New Ce3+ doped Ca2YMgScSi3O12 garnet ceramic phosphor for white LED converters - Khaidukov, Nicholas; Zorenko, Yu.; Zorenko, T.; Iskaliyeva, A.; Paprocki, Kazimierz; Zhydachevskii, Y.; Suchocki, Andrzej; Van Deun, R.; Batentschuk, Miroslaw (Physica Status Solidi-Rapid Research Letters, 2017)
The results on crystallization and investigation of the luminescent properties of a new prospective ceramic phosphor based on the Ce3+ doped Ca2YMgScSi3O12 silicate garnet are presented for the first time in this work. The luminescent properties of Ca2YMgScSi3O12:Ce were compared with the properties of the reference Ca3Sc2Si3O12:Ce ceramic sample. Without any doubt, the results of this research can be suitable for the development of a new generation of white converters based on the Ca2+ –Si4+ garnet compounds.<br
Keywords: cerium; doping; ceramics; silicates; garnets; phosphors

NEW PV SYSTEM CONCEPT –INDUCTIVE POWER TRANSFER FOR PV MODULES - Carigiet, Fabian; Knecht, Raphael; Baumann, Thomas; Brabec, Christoph; Baumgartner, Franz (35th European Photovoltaic Solar Energy Conference and Exhibition (35th EUPVSEC), 2018)
The proposed new PV system concept is based on several AC modules that are connected in series using inductive power transfer. These modules include a cell matrix thatis connected to a module integrated DC/AC inverter. The high frequency AC current flows through the primary side planar coil generating a magnetic flux. Outside of the PV module, there is a clamp including ferromagnetic material for the magnetic circuit that caries the magnetic flux to the secondary winding. The magnetic flux induces an AC current in the secondary winding,which is formed by the common cable.An AC/AC converter is placed at the end of the PV module strings to generate the 50Hz and to connect the PV power plant to the electricity grid. This new PV system concept is a fundamentally new approach of the electricity transmission in the field of PVsystem technology. It is not restricted to the replacement or optimisation of an individual system component, but it requires the continuing development of the PV module construction and the contactless connection technology to the common cable. The proposed inductive power transfer per each PV module opens up a complete new field for the PV system technolo
Keywords: PV system; inductive power transfer; AC modules; module integration

New silicate based thermographic phosphors Ca3Sc2Si3O12:Dy, Ca3Sc2Si3O12:Dy,Ce and their photoluminescence properties - Chepyga, Liudmyla M.; Osvet, Andres; Levchuk, Ievgen; Ali, Amjad; Zorenko, Yuriy; Gorbenko, Vitalii; Zorenko, Tetiana; Fedorov, Alexander; Brabec, Christoph; Batentschuk, Miroslaw (Journal of Luminescence, 2018)
We report on the investigation of Dy3+-doped calcium scandium silicate garnet (Ca3Sc2Si3O12 abbreviated CSSG) as a high-temperature thermographic phosphor. The Ca3Sc2Si3O12:Dy and Ca3Sc2Si3O12:Dy,Ce phosphors were synthesized: (1) as single crystalline films by liquid phase epitaxy on Gd3Ga3Al2O12 (GAGG) substrates using PbO-B2O3 flux; (2) via fatty acid-assisted co-precipitation method with subsequent annealing. The resulting silicate based phosphors CSSG:Dy, CSSG:Dy,Ce were characterized by XRD, SEM, photoluminescence excitation and emission spectroscopy. Temperature dependence of the emission spectra was studied at temperatures between 293 K and 1500 K. The emission in two spectral regions corresponding to the transitions in Dy3+ ions at 458 nm and 484 nm shows different temperature sensitivity allowing for thermometry based on the intensity ratio method. The intensity ratio and the sensitivity of single crystalline films are higher than those of co-precipitated materials at temperatures up to 1200 K. It is concluded that the CSSG:Dy is a promising phosphor for high temperature measurements, with its temperature sensitivity comparable to YAG:
Keywords: Cerium; Phosphor thermometry; Silicate garnet; Dysprosium; Luminescence

Noble metal free photocatalytic H2 generation on black TiO2: On the influence of crystal facets vs. crystal damage - Liu, Ning; Steinrueck, Hans-Georg; Osvet, Andres; Yang, Yuyun; Schmuki, Patrik (Applied Physics Letters, 2017)
In this study, we investigate noble metal free photocatalytic water splitting on natural anatase single crystal facets and on wafer slices of the [001] plane before and after these surfaces have been modified by high pressure hydrogenation and hydrogen ion-implantation. We find that on the natural, intact low index planes, photocatalytic H2 evolution (in the absence of a noble metal co-catalyst) can only be achieved when the hydrogenation treatment is accompanied by the introduction of crystal damage, such as simple scratching and miscut in the crystal, or by implantation damage. X-ray reflectivity, Raman, and optical reflection measurements show that plain hydrogenation leads to a ≈ 1 nm thick black titania surface layer without activity, while a colorless, density modified, and ≈7 nm thick layer with broken crystal symmetry is present on the ion implanted surface. These results demonstrate that (i) the H-treatment of an intact anatase surface needs to be combined with defect formation for catalytic activation and (ii) activation does not necessarily coincide with the presence of black color.<br

Noble-Metal-Free Photocatalytic Hydrogen Evolution Activity: The Impact of Ball Milling Anatase Nanopowders with TiH2 - Zhou, Xuemei; Liu, Ning; Schmidt, Jochen; Kahnt, Axel; Osvet, Andres; Romeis, Stefan; Zolnhofer, Eva; Marthala, Venkata; Guldi, Dirk Michael; Peukert, Wolfgang; Hartmann, Martin; Meyer, Karsten; et al. (Advanced Materials, 2016)
Ball milling TiO2 anatase together with TiH2 can create an effective photocatalyst. The process changes the lattice and electronic structure of anatase. Lattice deformation created by mechanical impact combined with hydride incorporation yield electronic gap-states close to the conduction band of anatase. These provide longer lifetimes of photogenerated charge carriers and lead to an intrinsic cocatalytic activation of anatase for H2 evolution.
Keywords: Anatase; Ball milling; Hydrogen evolution; Photocatalysis; Titanium hydride

Noncapacitive Hysteresis in Perovskite Solar Cells at Room Temperature - Almora Rodriguez, Osbel; Aranda, Clara; Zarazua, Isaac; Guerrero, Antonio; Garcia-Belmonte, Germà (ACS Energy Letters, 2016)
The current density-voltage (J-V) curves of perovskite solar cells have a hysteresis-like distortion. Hysteresis has raised many concerns about the feasibility and long-term stability of this kind of photovoltaic technology. However, there is a lack of distinction among different hysteretic phenomena which is necessary to unravel its underlying physical and chemical mechanisms. Here we distinguish between capacitive and noncapacitive currents giving rise to specific hysteretic responses in the J-V curves of PSCs. Hysteresis dominates in regular structures with TiO2 as bottom electron selective layer. This is due to the charge, both ionic and electronic, accumulation ability of the TiO2/perovskite interface but no influence on the steady-state operation. Noncapacitive hysteresis is observable at slow enough scan rates in all kind of architectures. Inverted structures, including organic compounds as a result of the inherent reactivity of contact materials and absorbers perovskites.<br

Nondestructive characterization of polymeric components of silicon solar modules by near-infrared absorption spectroscopy (NIRA) - Stroyuk, Oleksandr; Buerhop-Lutz, Claudia; Vetter, Andreas; Hauch, Jens; Brabec, Christoph (Solar Energy Materials and Solar Cells, 2020)
Degradation of the encapsulation of solar modules is a crucial problem and, accordingly, tools for assessing and understanding the according degradation is an important research topic. Here, we suggest near-infrared absorption spectroscopy (NIRA) as a tool that has a strong potential to determine non-invasively the composition of encapsulant and backsheet in typical commercial solar modules. Correct polymer identification by NIRA was supported by Raman spectral microscopy of the backsheet air sides and cross sections, with a hundred solar modules of different manufacturers analyzed by both spectroscopic methods. Furthermore, a specific NIRA measurement regime of the ethylene vinyl acetate encapsulant is presented, which potentially allows to probe the composition of the encapsulant non-destructively under lab conditions but also on silicon solar modules in the field installed outdoors. The present report shows NIRA to be a powerful method for non-destructive analysis of backsheets and encapsulants of photovoltaic silicon modules.
CRIS-Team Scopus Importer:2020-08-14
Keywords: Backsheet; Encapsulant; Near infrared spectroscopy; Photovoltaics; Raman spectroscopy; Silicon solar modules

Non-destructive imaging of defects in Ag-sinter die attach layers – A comparative study including X-ray, Scanning Acoustic Microscopy and Thermography - Dreher, P.; Schmidt, R.; Vetter, Andreas; Hepp, Johannes; Karl, André; Brabec, Christoph (Microelectronics Reliability, 2018)
In typical power electronic modules several semiconductor dies such as MOSFET or IGBT are soldered to a DBC substrate. During module production the quality of the solder layers can be monitored by the use of X-ray inspection and the void rate can be determined. Recently, the more robust Ag-sinter technology is deployed for attaching the power dies to the substrate, especially for high reliability or high temperature requirements. Besides voiding also adhesion problems can occur during sintering due to multiple reasons (e.g. contamination). In contrast to volume defects, pure adhesion problems cannot be detected by means of X-rays. Accordingly, other methods have to be applied for process monitoring. The present investigation compares the advantages and disadvantages of different non-destructive imaging techniques towards the detection of defects in sinter layers. Besides X-ray, Scanning Acoustic Microscopy (SAM) and Lock-in Thermography methods (DLIT+ILIT) were studied and evaluated in terms of suitability for detecting different defect types, resolution (minimum defect sizes), inspection time and possible integration into the assembly process.<br
Keywords: Lock-in thermography; Scanning acoustic microscopy; Silver-sintering technology; Non-destructive imaging; Power electronics; X-ray

Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells - Li, Renlong; Liu, Gongchu; Fan, Baobing; Du, Xiaoyan; Tang, Xiaofeng; Li, Ning; Ying, Lei; Brabec, Christoph; Huang, Fei; Cao, Yong (Organic Electronics, 2018)
We report two novel small-molecule non-fullerene acceptors, IF-TN and IDT-TN, with indenofluorene (IF) and indacenodithiophene (IDT) as their respective central electron-rich cores and naphthyl-fused indanone (N) as the electron-withdrawing end-groups. By pairing these non-fullerene acceptors with a widely used polymer PBDB-T, the fabricated polymer solar cells based on PBDB-T:IDT-TN obtain a power conversion efficiency up to 5.89%, associated with a relatively high open-circuit voltage of 0.97 V and a remarkably low non-radiative open-circuit voltage loss of 0.23 V. These findings provide an effective approach for the rational molecular design of small-molecule acceptors with low non-radiative open-circuit voltage los
Keywords: Open-circuit voltage loss; Small molecules; Electron acceptors; Non-fullerene polymer solar cells

Non-fused ring electron acceptors for high-performance and low-cost organic solar cells: Structure-function, stability and synthesis complexity analysis - Luo, Dou; Brabec, Christoph; Kyaw, Aung Ko Ko (Nano Energy, 2023)
In recent years, the development of non-fullerene acceptors (NFAs) has led to rapid advances in organic solar cells (OSCs), resulting in power conversion efficiency (PCE) exceeding 19%. However, the complex molecular architectures of fused ring electron acceptors require increasing steps of synthesis and purification, restricting their mass synthesis and hindering the commercialization of OSCs. Non-fused ring electron acceptors (NFREAs), on the other hand, offer several advantages, including facile synthesis, simple chemical modifications, and easy tunability in the optical band gap, frontier molecular orbitals and absorption spectrum. In the last five years, various types of NFREAs have been designed and synthesized, necessitating a timely review article. In this review, we analyze the molecular structure design and structure-property relationship of NFREAs, discuss their stability and figure-of-merit (FOM) values compared to traditional fused ring electron acceptors, and explore the challenges and prospects of NFREAs for achieving high-performance device and low-cost manufacturing concurrently.
CRIS-Team Scopus Importer:2024-12-16
Keywords: Figure-of-merit; Non-fused ring electron acceptors; Organic solar cell; Stability

Non-Stationary Outdoor EL-Measurements with a Fast and Highly Sensitive InGaAs Camera - Adams, Jens; Doll, Bernd; Buerhop-Lutz, Claudia; Pickel, Tobias; Teubner, Janine; Camus, Christian; Brabec, Christoph (32nd European Photovoltaic Solar Energy Conference and Exhibition, 2016)
A fast and non-destructive failure inspection is essential to ensure the reliability of PV plants. This might only be achieved by using highly sensitive imaging techniques which can detect local electrical losses in solar modules. Here, EL imaging used for outdoor PV inspection gained large interest, since it can be used to characterize local cell damages, series resistances, and shunts in solar modules. However, currently available EL-cameras base on Si-CCD-detectors. Besides high spatial resolutions they exhibit long image integration times (IIT) of about 1 s for obtaining evaluable EL-images. To overcome this limitation we present, for the first time, an InGaAs camera which enables a complete EL-characterization of field-installed PV-modules with IITs between 1 ms and 20 ms. The short IITs result in frame rates of about 100 Hz which even offer the opportunity of recording dynamic EL-movies and daylight Electroluminescence Lock-In (ELLI) measurements. To test the outdoor applicability we compared ELmovies of the InGaAs camera with EL-images of a Si-CCD camera and show that all relevant power reducing cell failures (e. g. cracks) can be detected and characterized.
Keywords: Performance and Reliability measurement; Electroluminescence of PV Modules; Energy rating; Degradation; Ageing; Lifetime Characterization of PV Cells and Modules

Non-Uniformity Measurements of a Steady State Solar Simulator Using the Hishikawa-Hashimoto Method and Subsequent Improvement - Doll, Bernd; Hoyer, Ulrich; Hofer, Markus; Pickel, Tobias; Buerhop-Lutz, Claudia; Camus, Christian; Hauch, Jens; Brabec, Christoph (32nd European Photovoltaic Solar Energy Conference and Exhibition, 2016)
A steady state solar simulator has been set up at the ZAE Bayern. The system is meant to be used for the characterization of photovoltaic (PV) of various technologies. In particular, it can be used for pre-conditioning of thin film modules in order to enable reliable power and I-V measurements. Furthermore, it allows for determining the temperature coefficient of PV modules of various technologies. The latter application will also be discussed in this work. Here, the optimization of the steady state solar simulator based on a method originally introduced by Hishikawa et al. for pulsed solar simulators is described and the results of this optimization are presented.
Keywords: Characterization; Experimental Methods; Light-Soaking; Qualification and Testing

Novel fatty-acid assisted co-precipitation method for the synthesis of SrAl2O4: Eu2+, Sm3+ storage phosphor nanoparticles - Romling, Lukas; Levchuk, Ievgen; Steimle, Max; Osvet, Andres; Batentschuk, Miroslaw; Winnacker, Albrecht; Brabec, Christoph (Advanced Materials - TechConnect Briefs 2017, 2017)
In this work SrAl2O4 doped with Eu2+ and Sm3+ were successfully synthesized using a novel fatty acid assisted co-precipitation method. The main focus was laid on (1) finding a suitable synthesis procedure to reproducibly obtain nanosized SrAl2OM4 doped with Europium and Samarium of high purity (2) optimizing the Eu2+ concentration for nano scaled Strontium Aluminate and finally (3) optimizing the co-doping amount of Sm3+ to obtain a high intensity photostimulated luminesence (PSL). As a last step the dependence of the PSL signal to the writein and read-out wavelengths of the stimulating light was determined.
Keywords: Co-precipitation; Phosphor; Photostimulated luminescence; SrAl2O4

Novel Photonic Applications of Silicon Carbide - Ou, Haiyan; Shi, Xiaodong; Lu, Yaoqin; Kollmuß, Manuel; Steiner, Johannes; Tabouret, Vincent; Syväjärvi, Mikael; Wellmann, Peter; Chaussende, Didier (Materials, 2023)
Silicon carbide (SiC) is emerging rapidly in novel photonic applications thanks to its unique photonic properties facilitated by the advances of nanotechnologies such as nanofabrication and nanofilm transfer. This review paper will start with the introduction of exceptional optical properties of silicon carbide. Then, a key structure, i.e., silicon carbide on insulator stack (SiCOI), is discussed which lays solid fundament for tight light confinement and strong light-SiC interaction in high quality factor and low volume optical cavities. As examples, microring resonator, microdisk and photonic crystal cavities are summarized in terms of quality (Q) factor, volume and polytypes. A main challenge for SiC photonic application is complementary metal-oxide-semiconductor (CMOS) compatibility and low-loss material growth. The state-of-the-art SiC with different polytypes and growth methods are reviewed and a roadmap for the loss reduction is predicted for photonic applications. Combining the fact that SiC possesses many different color centers with the SiCOI platform, SiC is also deemed to be a very competitive platform for future quantum photonic integrated circuit applications. Its perspectives and potential impacts are included at the end of this review paper.
CRIS-Team Scopus Importer:2023-02-24
Keywords: integrated photonics; material growth; silicon carbide

Novel two-dimensional phosphor thermography by decay-time method using a low frame-rate CMOS camera - Ali, Amjad; Chepyga, Liudmyla; Khanzada, Laraib Sarfraz; Osvet, Andres; Brabec, Christoph; Batentschuk, Miroslaw (Optics and Lasers in Engineering, 2020)

Novel wide-bandgap non-fullerene acceptors for efficient tandem organic solar cells - Firdaus, Yuliar; He, Qiao; Lin, Yuanbao; Nugroho, Ferry Anggoro Ardy; Le Corre, Vincent Marc; Yengel, Emre; Balawi, Ahmed H; Seitkhan, Akmaral; Laquai, Frederic; Langhammer, Christoph; Liu, Feng; Heeney, Martin; et al. (Journal of Materials Chemistry A, 2020)
The power conversion efficiency (PCE) of tandem organic photovoltaics (OPVs) is currently limited by the lack of suitable wide-bandgap materials for the front-cell. Here, two new acceptor molecules, namely IDTA and IDTTA, with optical bandgaps (Eoptg) of 1.90 and 1.75 eV, respectively, are synthesized and studied for application in OPVs. When PBDB-T is used as the donor polymer, single-junction cells with PCE of 7.4%, for IDTA, and 10.8%, for IDTTA, are demonstrated. The latter value is the highest PCE reported to date for wide-bandgap (Eoptg >= 1.7 eV) bulk-heterojunction OPV cells. The higher carrier mobility in IDTTA-based cells leads to improved charge extraction and higher fill-factor than IDTA-based devices. Moreover, IDTTA-based OPVs show significantly improved shelf-lifetime and thermal stability, both critical for any practical applications. With the aid of optical-electrical device modelling, we combined PBDB-T:IDTTA, as the front-cell, with PTB7-Th:IEICO-4F, as the back-cell, to realize tandem OPVs with open circuit voltage of 1.66 V, short circuit current of 13.6 mA cm(-2) and a PCE of 15%; in excellent agreement with our theoretical predictions. The work highlights IDTTA as a promising wide-bandgap acceptor for high-performance tandem OPVs.

Nucleation and growth of polycrystalline SiC - Kaiser, Michl; Schimmel, Saskia; Jokubavicius, Valdas; Linnarsson, Margareta K.; Ou, Hayan; Syväjärvi, Mikael; Wellmann, Peter (IOP Conference Series: Materials Science and Engineering, 2014)
The nucleation and bulk growth of polycrystalline SiC in a 2 inch PVT setup using isostatic and pyrolytic graphite as substrates was studied. Textured nucleation occurs under near-thermal equilibrium conditions at the initial growth stage with hexagonal platelet shaped crystallites of 4H, 6H and 15R polytypes. It is found that pyrolytic graphite results in enhanced texturing of the nucleating gas species. Reducing the pressure leads to growth of the crystallites until a closed polycrystalline SiC layer containing voids with a rough surface is developed. Bulk growth was conducted at 35 mbar Ar pressure at 2250 degrees C in diffusion limited mass transport regime generating a convex shaped growth form of the solid-gas interface leading to lateral expansion of virtually [001] oriented crystallites. Growth at 2350 degrees C led to the stabilization of 6H polytypic grains. The micropipe density in the bulk strongly depends on the substrate use
Keywords: fluorescent SiC; PVT; nucleation; polycristalline growth

Nucleation Control of Cubic Silicon Carbide on 6H-Substrates - Vasiliauskas, Remigijus; Marinova, Maya; Hens, Philip; Wellmann, Peter; Syvajarvi, Mikael; Yakimova, Rositza (Crystal Growth & Design, 2012)
The nucleation of cubic (3C) SiC on on-axis 6H-SiC was investigated in the temperature range 1500-1775 degrees C by the technique of sublimation epitaxy. We have studied two different cases: (i) the initial homoepitaxial growth of 6H-SiC followed by nucleation of 3C-SiC and (ii) nucleation of homoepitaxial 6H-SiC islands. The supersaturation in the growth cell was calculated using the modeled source to substrate temperature difference. We show that, at low temperature and supersaturation, growth of 6H-SiC commences in spiral growth mode, which prepares the surface for 3C-SiC nucleation. Provided the supersaturation is high enough, the 3C-SiC nucleates as two-dimensional islands on terraces of the homoepitaxial 6H-SiC. Detailed structural study indicates that the 3C-SiC began to grow on defect free surfaces. From the experimental and modeling results, we show that the growth parameter window for 3C-SiC is rather narrow. Deviation from it can result in 6H-SiC growth in spiral or 2D-nucleation mode, which suggests the importance of knowledge of supersaturation.
Keywords: Silicon Carbide; Cubic SiC; 6H substrate; nucleation control

Numerical modeling and experimental verification of modified-PVT crystal growth of SiC - Wellmann, Peter; Pons, Michel (Journal of Crystal Growth, 2007)
This work focuses on numerical modeling of temperature field and mass transport in a so-called modified physical vapor transport (M-PVT) growth configuration, which is a further development of the state-of-the-art SiC bulk crystal growth technique. Modeling has been carried out by adapting the commercial computer code CFD-ACE+ (CFD Research Corporation) to the special application. In particular, linked to experimental results in the own lab, we will first show that modeling may be applied to numerically reproduce the impact of an additional gas inlet on the resulting effective mass transport inside the growth setup. In a second step, we will use modeling to understand the physical and chemical processes during crystal growth. We will show how additional gases may be used to tailor the gas phase composition, i.e. C/Si ratio, in front of the SiC growth interface. (c) 2007 Elsevier B.V. All rights reserved.
Keywords: Numerical modeling; Crystal growth; M-PVT; SiC

Numerical reactive diffusion modeling of stacked elemental layer rapid thermal annealed chalcopyrite absorber layer formation - Zweschke, André; Wellmann, Peter (Thin Solid Films, 2015)
The diffusion processes during chalcopyrite (CuInSe) layer formation by rapid thermal annealing (RTA) of stacked elemental layers are still not completely understood. We present a one-dimensional kinetic model, which describes the diffusive reaction kinetics for layers of partial miscible systems to describe the RTA process. The model was applied to a layer stack of elemental Se between the metallic layers Cu and In. First, some binary phases occur. After the Se layer is consumed, CuInSeis observed between the binary compounds CuSe and InSe.
Keywords: Chalcopyrite; Reactive diffusion; Diffusion modeling

Numerical Simulation of light propagation in silver nanowire films using time-harmonic inverse iterative method - Yan, Shuai; Krantz, Johannes; Forberich, Karen; Pflaum, Christoph; Brabec, Christoph (Journal of Applied Physics, 2013)
The interaction between light and silver nanowires (Ag NWs) in a thin film is simulated by solving Maxwells equations numerically. Time-harmonic inverse iterative method is implemented to overcome the problem of negative permittivity of silver, which makes the classical finite-difference time-domain iteration unstable. The method is validated by showing the correspondence between the plasmonic resonance of an Ag NW from a two dimensional simulation and the analytical solution. In agreement with previous experimental studies, the simulation results show that the transmissivity of the Ag NW films is higher than expected from the geometric aperture. The cause of this phenomenon is studied using TE/TM modes analysis for Ag NW films with different surface coverage of parallel-aligned Ag NWs. Furthermore, 3D simulation of Ag NW films with randomly arranged Ag NWs is performed by parallel computation on high performance computers. A binder layer is taken into account for a preliminary comparison between the simulation and experimental results. The agreements and disagreements between the simulated and measured spectra are discussed. © 2013 AIP Publishing LLC.
UnivIS-Import:2015-03-09:Pub.2013.tech.IMMD.c3sim.numeri
Keywords: Finite difference time domains; High performance computers; Negative permittivity; Parallel Computation; Plasmonic resonances; Silver nanowires; Surface coverages; Two-dimensional simulations Engineering controlled terms: Computer simulation; Electric resistance measurement; Finite difference time domain method; Inverse problems; Iterative methods Engineering main heading: Silver

Numerical simulation of SIC processes: A characterization tool for the design of epitaxial structures in electronics - Pons, Michel; Nishizawa, Shin-Ichi; Wellmann, Peter; Ucar, M.; Blanquet, E.; Dedulle, J.M.; Baillet, F.; Chaussende, D.; Bernard, C.; Madar, R. (ECS Transactions, 2005)
Modeling and simulation of the SiC growth processes, Physical Vapor Transport (PVT) and Chemical Vapor Deposition (CVD), are sufficiently mature to be used as a training tool for engineers as well as a growth machine design tool, e.g. when building new process equipment or up-scaling old ones. It is possible to simulate accurately temperature and deposition distributions, as well as doping. The key of success would be the combined use of simulation, experiments and characterization in a "daily interaction". The different presented examples have the aim to show that this approach has the potential of a characterization tool which could be of great importance in the optimization of epitaxial structures used for the fabrication of SiC-based devices.
Keywords: SiC growth; modeling; simulation

Numerical simulation of thermal stress formation during PVT-growth of SiC bulk crystals - Selder, Markus; Kadinski, Lev; Durst, Franz; Straubinger, Thomas; Wellmann, Peter; Hofmann, Heinz-Dieter (Materials Science Forum, 2001)
The thermal stress distribution in growing SiC bulk single crystals is analyzed by a Finite Volume solution approach using anisotropic elasticity theory. The stress calculations are based on a global simulation of heat and mass transfer during the SiC bulk growth process. The temporal evolution of the thermal stress distribution inside the growing crystal is studied. It is found that the conditions for thermal stress formation at fixed positions in the crystal vary significantly during growth. The impact of the stress boundary conditions (free/fixed surfaces) on this behaviour is investigated. The calculated stresses exceed considerably the critical resolved shear stress in SiC which indicates that the observed dislocation formation under the established growth conditions should be caused at least partly by thermal stresse
UnivIS-Import:2015-10-26:Pub.2001.tech.ITC.stmmec.numeri_7
Keywords: Bulk Growth; Global Heat and Mass Transfer; Numerical Modeling; Physical Vapor Transport; Thermoelastic Stress Distribution

Numerical Simulations of Ammonothermal Crystal Growth of GaN and Pathways towards their Experimental Validation - Schimmel, Saskia; Tomida, Daisuke; Saito, Makoto; Bao, Quanxi; Ishiguro, Tohru; Honda, Yoshio; Chichibu, Shigefusa F.; Amano, Hiroshi; Wellmann, Peter (Seminar of the Young Crystal Growers (DGKK), 2021)

Numerical study of plasmonic absorption enhancement in semiconductor absorbers by metallic nanoparticles - Hornich, Julian; Pflaum, Christoph; Brabec, Christoph; Forberich, Karen (Journal of Applied Physics, 2016)
We are studying the influence of spherical silver nanoparticles (AgNP) in absorbing media by numerically solving the Maxwell's equations. Our simulations show that the near-field absorption enhancement introduced by a single AgNP in the surrounding medium is increasing with the growing particle diameter. However, we observe that the relative absorption per particle volume is on a similar level for different particle sizes; hence, different numbers of particles with the same total volume yield the same near-field absorption enhancement. We also investigate the effect of non-absorbing shells around the AgNP with the conclusion that even very thin shells suppress the beneficial effects of the particles noticeably. Additionally, we include AgNP in an organic solar cell at different vertical positions with different particle spacings and observe the beneficial effects for small AgNP and the scattering dependent performance for larger particles.

Observation of lattice plane bending during SiC PVT bulk growth using in-situ high energy x-ray diffraction - Hock, Rainer; Konias, Katja; Perdicaro, Laura Maria Stella; Magerl, Andreas; Hens, Philip; Wellmann, Peter (Materials Science Forum, 2010)
We have investigated thermally induced strain in the SiC crystal lattice during physical vapor transport bulk growth. Using high energy x-ray diffraction lattice plane bending was observed in-situ during growth. With increasing growth rate increasing lattice plane bending and, hence, strain was observed. A comparison with numerical modeling of the growth process shows that the latter is related to the heat of crystallization which needs to be dissipated from the crystal growth front. The related temperature gradient as driving force for the dissipation of the heat of crystallization causes lattice plane bending. Optimization of the growth process needs to consider such effect
Keywords: Bulk Crystal Growth; PVT Growth

Observation of reversible light degradation in organic photovoltaics induced by long-persistent radicals - Zhang, Difei; Liu, Chao; Zhang, Kaicheng; Jia, Yanhua; Zhong, Wenkai; Qiu, Weidong; Li, Yuanfeng; Heumüller, Thomas; Forberich, Karen; Le Corre, Vincent Marc; Lüer, Larry; Li, Ning; et al. (Energy and Environmental Science, 2023)
With the rapid development of organic photovoltaics, device stability has become a crucial obstacle hindering their transition from laboratory-scale to industrial applications. However, it still remains unclear how light differs from heat in driving trap formation and device degradation. On the basis of the PTzBI-dF:Y6-BO system, it is observed that the post-thermal annealing on these high-performance organic solar cells can partially recover the light-induced burn-in losses. The recovery process is found to be correlated with a reversible charge extraction ability, reversible trap density of state, local charge carrier density and charge accumulation. Herein, we propose an innovative mechanism for light degradation in organic photovoltaic devices, which is triggered by the presence of light-induced long-persistent radicals. The findings offer deep insights into light degradation of organic photovoltaics and a new perspective for improving device stability under long-term operation.
CRIS-Team Scopus Importer:2023-12-01

Oligomer-Assisted Photoactive Layers Enable >18 % Efficiency of Organic Solar Cells - Cheng, Yujun; Huang, Bin; Huang, Xuexiang; Zhang, Lifu; Kim, Seoyoung; Xie, Qian; Liu, Chao; Heumüller, Thomas; Liu, Zuoji; Zhang, Youhui; Wu, Feiyan; Yang, Changduk; et al. (Angewandte Chemie International Edition, 2022)
Although ternary organic solar cells (OSCs) have unique advantages in improving device performance, the morphology assembly in the ternary-phase would be more uncertain or complex than that in the binary-phase. Here, we propose a new concept of oligomer-assisted photoactive layers for high-performance OSCs. The formed alloy-like phase of the oligomer : host polymer blend enabled the oligomer-assisted OSCs to fuse the advantages of both binary and ternary devices, exhibiting substantially enhanced performance and stability compared to the control devices. With the addition of oligomers, outstanding efficiencies of 17.33 % for a PM6 : Y6 device, 18.32 % for a PM6 : BTP-eC9 device, and 17.13 % for a PM6/Y6 pseudo-bilayer device were achieved, all of which are one of the highest values in their corresponding fields. The improved performance originated from the downshift energy levels, enhanced light absorption, optimized blend morphology, favorable charge dynamics, and reduced non-radiative energy loss.
CRIS-Team Scopus Importer:2022-04-01
Keywords: Organic Solar Cells; Performance; Stability

One pot menthol synthesis via hydrogenations of citral and citronellal over montmorillonite-supported Pd/Ni-heteropoly acid bifunctional catalysts - Shah, Abdul Karim; Maitlo, Ghulamullah; Shah, Aqeel Ahmed; Channa, Iftikhar Ahmed; Kandhro, Ghulam Abbas; Maitlo, Hubdar Ali; Bhatti, Umair Hassan; Shah, Ahmed; Memon, Abdul Qayoom; Jatoi, Abdul Sattar; Park, Yeung Ho (Reaction Kinetics Mechanisms and Catalysis, 2019)
Menthol synthesis is possible through citral and citronellal hydrogenations via following multistage chemical reactions such as hydrogenation and cyclization. This research mainly focuses on a design of selective, active and cost-effective metal-acid (bifunctional) catalysts for menthol production via citral and citronellal hydrogenations. More specifically, Pd and Ni metals were impregnated over acidic support (e.g., hetero-poly acid supported montmorillonite, HPA_MM). The prepared catalysts were characterized by BET, pyridine adsorption and amine titration methods. Some of the most important parameters such as metal type and loading; applied pressure and reaction time have been investigated throughout this work. The obtained results reveals that the 8 wt% Ni-HPA-MM catalyst (Cat-5) has produced 63% menthols (initial reaction rate 0.126mmol/min) from citral hydrogenation (80 degrees C, 1.0MPa) within 24h. Similarly, during lower applied pressure (0.5MPa), the production of menthol was significantly improved (approximately 98% of menthol, initial reaction rate similar to 0.138mmol/min) with the application of 8 wt% Ni-HPA-MM catalyst (Cat-5) in citronellal hydrogenation. Higher menthol selectivity was achieved from both reactions (citral and citronellal hydrogenation) which might be due to the presence of strong Lewis and medium BrOnsted acid sites.
CRIS-Team WoS Importer:2019-10-18

Online monitoring of PVT SiC bulk crystal growth using digital x-ray imaging - Wellmann, Peter; Bickermann, Matthias; Grau, M.; Hofmann, Heinz-Dieter; Straubinger, Thomas; Winnacker, Albrecht (Materials Research Society Symposium - Proceedings, 1999)
An advanced method based on x-ray imaging is presented which allows us to visualize the ongoing processes during physical vapor transport (PVT) growth of SiC. Using a high resolution and high speed x-ray imaging detector based on image plates and digital recording we are able to follow the SiC bulk single crystal growth as well as the evolution of the SiC powder source inside the inductively heated graphite crucible on-line and quasi-continuously.

On Mott-Schottky Analysis Interpretation of Capacitance Measurements in Organometal Perovskite Solar Cells - Almora Rodriguez, Osbel; Aranda, Clara; Mas-Marza, Elena; Garcia-Belmonte, Germà (Applied Physics Letters, 2016)
Capacitance response of perovskite-based solar cells (PSCs) can be exploited to underlying physical mechanisms, both in bulk and at outer interfaces. Especially interesting is the use of the depletion layer capacitance theory to PSCs, following common procedures used with inorganic and organic photovoltaic devices. Voltage modulation of the depletion layer allows extracting relevant parameters as the absorber defect density and built-in potential by means of the Mott-Schottky (MS) analysis. However, the uncritical use of the MS technique, commonly observed in PSCs, produces on the measured capacitance value. Avoiding accumulation capacitive parasitic contributions. It is noted that the distinction of capacitance is only feasible in the case of perovskite layers containing significant defect density ( 1017 cm-3). It is confirmed that MS is reproducible by hysteresis reduction at slow scan rates, and positive bias starting polarization. A complete procedure with specific checking points is provided here for consistent MS measurement and interpretation.
Keywords: capacitance measurement; organic compounds; solar cells

On the critical competition between singlet exciton decay and free charge generation in non-fullerene based organic solar cells with low energetic offsets - Pranav, Manasi; Shukla, Atul; Moser, David; Rumeney, Julia; Liu, Wenlan; Wang, Rong; Sun, Bowen; Smeets, Sander; Tokmoldin, Nurlan; Cao, Yonglin; He, Guorui; Beitz, Thorben; et al. (Energy and Environmental Science, 2024)
Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets - particularly the relationship between ionization energy offset (ΔIE) and free charge generation. Here, we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights, covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a ΔIE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small ΔIE blends, which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells, with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE-CT hybridization. In agreement with this conclusion, transient absorption spectroscopy consistently reveals that a smaller ΔIE slows the NFA exciton dissociation into free charges, albeit restorable by an electric field. Our experimental data align with Marcus theory calculations, supported by density functional theory simulations, for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE, but that this restriction is lifted in systems with a small reorganization energy for charge transfer.
CRIS-Team Scopus Importer:2024-10-04

On the excitation mechanism of erbium and ytterbium in the quaternary compounds InGaAsP - Wellmann, Peter; Winnacker, Albrecht; Pensl, Gerhard (Materials Research Society Symposium - Proceedings, 1996)
An efficient transfer of excitonic energy to rare earth (RE) ions is crucial for possible optoelectronic applications of RE doped semiconductors. In order to investigate the energy transfer mechanism to RE ions after optical above bandgap excitation we studied the intensity of the 4I13/2 →4 4I15/2-transition of Er3+ (1.54μm) and the one of the 2F5/2 →2 F7/2 -transition of Yb3+ (lμm) (loped into InGaAsP-layers lattice matched to InP. By varying the composition of the quaternary compoun ds, the bandgap energy together with the RE bound exciton energy were tuned relative to the RE excitation energy, and the effect on the RE luminescence intensity was observed. The results can be interpreted by stating a) that the energy transfer to the RE proceeds via the RE bound exciton, and b) that the intensity of the RE luminescence is essentially determined by the rate of back transfer of the RE excitation energy ERE to the bound exciton (with excitation energy Ebe). In this back transfer the energy of the excited RE ion plus the energy of 0, 1, 2 ... L0-phonons (energy Elo) is used to reexcite the bound exciton, instead of being emitted as an RE luminescence photon. For compositions where Ebe = ERE + n. Elo (n = 0, 1, 2...) we have a maximum of back transfer and correspondingly a, minimum in RE luminescence. In between the intensity has a, maximu

On the importance of dislocation flow in continuum plasticity models for semiconductor materials - Nguyen, Binh Duong; Rausch, Alexander; Steiner, Johannes; Wellmann, Peter; Sandfeld, Stefan (Journal of Crystal Growth, 2020)
Crystalline defects, such as dislocations, may have a strong negative influence on the performance and quality of electronic devices. Modelling the defect evolution of such systems helps to understand where defects occur, how they evolve and interact, Modelling also might allow to avoid – or at least to reduce – the number of line defects in such materials. A widely used continuum model for predicting the evolution of dislocation density in semiconductors is the Alexander-Haasen (AH) model (Alexander and Haasen, Solid state physics,1969) which describes the evolution of dislocation density through a local dislocation multiplication law; the motion of dislocations, i.e. dislocation flow, is not considered. Within this work, the underlying assumptions for the validity of the AH model are studied and compared to a more complex continuum model of dislocation dynamics (CDD) which explicitly considers dislocation fluxes. We use a simplified 2D scenario of a 4H-SiC crystal at the end of the growth as a benchmark system. Our comparisons show that considering the motion of dislocations can have a significant influence on the evolution and final distribution of dislocation density, which may strongly limit the applicability of the AH model. Based on the CDD model, we then investigate the cooling process and study the effect of the cooling rate on the resulting dislocation microstructure.
CRIS-Team Scopus Importer:2019-12-27
Keywords: A1. Alexander-Haasen model; A1. Computer simulation; A1. Line defects; A1. Stresses; B2. Semiconducting materials

On the lattice parameters of silicon carbide - Stockmeier, Matthias; Müller, Robin; Sakwe, Aloysius; Wellmann, Peter; Magerl, Andreas (Journal of Applied Physics, 2009)
The thermal expansion coefficients of the hexagonal SiC polytypes 4H and 6H and with Al and N dopants have been determined for temperatures between 300 and 1770 K. Further, a set of the room temperature lattice parameters in dependence on doping with N, Al, and B has been obtained. Data for the thermal expansion were taken on a triple axis diffractometer for high energy x rays with a photon energy of 60 keV, which allows the use of large single crystals with a volume of at least 6x6x6 mm(3) without the need to consider absorption. The room temperature measurements for samples with different dopants have been performed on a four-circle diffractometer. The thermal expansion coefficients along the a- and c-directions, alpha(11) and alpha(33), increase from 3x10(-6) K(-1) at 300 K to 6x10(-6) K(-1) at 1750 K. It is found that alpha(11) and alpha(33) are isotropic within 10(7) K(-1). At high temperatures both coefficients for doped samples are similar to 0.2x10(-6) and 0.3x10(-6) K(-1) lower than for the undoped material.
Keywords: Silicon Carbide; 4H SiC; 6H SiC; thermal expansion coefficients; x-ray diffraction

On the origin of the below band-gap absorption bands in n-type (N) 4H-and 6H-SiC - Weingärtner, Roland; Wellmann, Peter; Winnacker, Albrecht (Materials Science Forum, 2004)
The dopant concentration dependence of the below band-gap absorption bands at room temperature in n-type (N) 4H- and 6H-SiC are investigated. The occupation of the nitrogen donors are determined from temperature dependent Hall effect measurements. In combination with the strength of the absorption bands the cross sections of these transitions are obtained. It is shown that the below band-gap absorption bands may be attributed to the inequivalent lattice sites of the nitrogen donor, i.e. the transition is defect-like with different initial states of the nitrogen donors and a common final state in the conduction ban
Keywords: Absorption Band; Cross Section; Hall-Effect; Inequivalent Lattice Sites; Nitrogen Donor

On the preparation of semi-insulating SiC bulk crystals by the PVT technique - Bickermann, Matthias; Hofmann, Heinz-Dieter; Straubinger, Thomas; Weingärtner, Roland; Wellmann, Peter; Winnacker, Albrecht (Applied Surface Science, 2001)
Vanadium in SiC can act as a deep acceptor compensating residual nitrogen (DeltaE approximate to 0.8 eV) or as a deep donor compensating p-type (Al/B) impurities (DeltaE approximate to 1.6 eV) leading to semi-insulating behavior. In any case, doping homogeneity is crucial. Additionally, the V solubility limit in SiC must not be exceeded. To determine impurity incorporation, growth of nominally undoped crystals was performed. Here, nitrogen is the residual impurity and the charge carrier concentration n is decreasing exponentially with growth time. Wafers with remaining n = 8 x 10(15) cm(-3) were obtained. During B-doped growth, the hole concentration increases with growth time. The influence of the compensation by nitrogen and the loss of boron content in the source is discussed. While effective boron segregation on the facet during growth is determined to be close to unity, this value is not reached due to evaporation of the B source. V incorporation is related to the partial pressure of the V species in the crucible. Several regions were found in V-doped crystals. V exceeding the solubility limit leads to the formation of V-rich precipitates, V source depletion is indicated by a change to n-type conducting behavior because of residual nitrogen. With temperature-dependent Hall effect measurements, the specific resistivity at room temperature is determined to rho (293K) = 2-5 x 10(11) Omega cm, while resistivity mappings show dopant inhomogeneities. By reducing the V species evaporation rate, bulk SiC crystals exhibiting precipitate-free, semi-insulating behavior were obtained. (C) 2001 Elsevier Science B.V. All rights reserved.
Keywords: Silicon carbide; Bulk growth; Boron doping; Vanadium doping; Dopant inhomogeneities

On the theoretical framework for meniscus-guided manufacturing of large-area OPV modules - Gumpert, Fabian; Janßen, Annika; Basu, Robin; Brabec, Christoph; Egelhaaf, Hans-Joachim; Lohbreier, Jan; Distler, Andreas (Progress in Organic Coatings, 2024)
For the manufacturing of thin films of solution-processable organic semiconductors, e.g. for organic photovoltaics (OPV), meniscus guided-coating techniques are the method of choice for large-scale industrial applications. However, the process requires an in-depth understanding of the respective fluid dynamics to control the resulting film thickness. In this article, we derive an analytical expression to describe the layer thickness of coatings manufactured with a trapezoidal-shaped applicator as a function of various fluid and process parameters. The analytical calculations are compared with results from computational fluid dynamics (CFD) simulations and experimental data for an industrially relevant OPV active material system. The good agreement of all three approaches demonstrates the potential of the analytical and simulative methods to minimize the number of time- and resource-consuming experiments. Furthermore, our theoretical model can be used to enhance the homogeneity of large-area coatings by means of an acceleration profile of the applicator that can compensate for the liquid loss during the coating process. The respective analytical expression is validated by simulated and experimentally obtained data for long-distance coatings. Finally, this approach is used to fabricate a large-area OPV module with new world record efficiency.
CRIS-Team Scopus Importer:2024-05-24
Keywords: Accelerated coating velocity; Analytical description; Computational fluid dynamics; Meniscus-guided coating; Organic photovoltaics; Thin films

Optical mapping of aluminum doped p-type SiC wafers - Wellmann, Peter; Straubinger, Thomas; Künecke, Ulrike; Müller, Ralf; Sakwe, Aloysius; Pons, Michel; Thuaire, A.; Crisci, A.; Mermoux, M.; Auvray, L.; Camassel, J. (physica status solidi (a), 2005)
We discuss the application of optical techniques to address the spatial distribution of electronic properties of highly aluminum doped p-type SiC wafers; optical techniques are superior over their electrical counterparts in a sense that they are non-destructive. While absorption and birefringence mapping are powerful tools to determine the homogeneity of charge carrier concentration and defects in n-type SiC, respectively, the same methods fail in highly p-type doped SiC due to the opaque nature of the latter. Therefore reflective methods like Raman spectroscopy and low temperature photoluminescence have to be applied in order to address electronic properties by optical techniques. (c) 2005 WILEY-VCH Verlag GmbH W Co. KGaA, Weinheim.
Keywords: Silicon carbide; charge carrier concentration; Al doping; optical determination; mapping

Optical model for simulation and optimization of luminescent down-shifting layers filled with phosphor particles for photovoltaics - Lipovsek, Benjamin; Solodovnyk, Anastasiia; Forberich, Karen; Stern, Edda; Krc, Janez; Brabec, Christoph; Topic, Marko (Optics Express, 2015)
We developed an optical model for simulation and optimization of luminescent down-shifting (LDS) layers for photovoltaics. These layers consist of micron-sized phosphor particles embedded in a polymer binder. The model is based on ray tracing and employs an effective approach to scattering and photoluminescence modelling. Experimental verification of the model shows that the model accurately takes all the structural parameters and material properties of the LDS layers into account, including the layer thickness, phosphor particle volume concentration, and phosphor particle size distribution. Finally, using the verified model, complete organic solar cells on glass substrate covered with the LDS layers are simulated. Simulations reveal that an optimized LDS layer can result in more than 6% larger short-circuit current of the solar cell.
CRIS-Team Scopus Importer:2023-03-07

Optical Model for Simulation and Optimization of Luminescent down-shifting Layers in Photovoltaics - Lipovsek, Benjamin; Solodovnyk, Anastasiia; Forberich, Karen; Stern, Edda; Brabec, Christoph; Krc, Janez; Topic, Marko (E-MRS Spring Meeting 2015 Symposium C - Advanced inorganic materials and structures for photovoltaics, 2015, 2015)
We present an optical model that was developed for the purpose of simulation and optimization of luminescent down-shifting layers based on phosphor particles. The model combines three-dimensional ray tracing with a novel effective scattering approach based on the Mie scattering theory. Verification of the model with experimental results shows that the model accurately takes various layer parameters into account, such as the layer thickness, the phosphor volume concentration, and the phosphor particle size distribution. Finally, using the verified model, we investigate the concept of luminescent down-shifting in organic solar cells and discuss the optimal layer parameters. Preliminary simulation results show that in the case of the studied solar cell, an improvement in the short-circuit current density of more than 6% can be obtained.
Keywords: Fluorescent and luminescent materials; Optical modelling and simulation; Solar energy; Wavelength conversion devices

Optical quantitative determination of doping levels and their distribution in SiC - Wellmann, Peter; Weingärtner, Roland; Bickermann, Matthias; Straubinger, Thomas; Winnacker, Albrecht (Materials Science and Engineering B-Advanced Functional Solid-State Materials, 2002)
We report the development of an absorption measurement-based characterization tool for the quantitative determination of doping levels and their lateral distribution in silicon carbide wafers. Calibration plots for the technologically important silicon carbide polytypes 4H-SiC (n-/p-type), 6H-SiC (n-/p-type) and 15R-SiC (n-type) are presented. A review of the underlying physical effects of the measurement procedure as well as a description of the experimental setup is given. The applicability of the characterization tool as a production friendly non-contact wafer quality test is demonstrated by showing several mappings of the lateral doping level distribution. The accuracy of the described measurement procedure is typically 15-20% and is of the same order as its electrical Hall measurement counterpart. (C) 2002 Elsevier Science B.V. All rights reserved.
Keywords: Silicon carbide; Optical properties; Doping effects; Electrical measurements

Optimising the parameters for the synthesis of CuIn-nanoparticles by chemical reduction method for chalcopyrite thin film precursors - Schuster, Matthias; Möckel, Stefan; Wibowo, Rachmat Adhi; Hock, Rainer; Wellmann, Peter (Materials Research Society Symposium - Proceedings, 2013)
Roll-to-roll deposition techniques for the fabrication of chalcopyrite solar cells are of major interest and are a promising alternative to state of the art vacuum processes. However, for roll-to-roll processes the preparation of precursor materials like nanoparticle inks is a crucial point. In this work a study on the preparation technique of copper-indium intermetallic nanoparticles was conducted. The preparation of the nanoparticles is based on the chemical reduction of copper and indium cations with sodium borohydride. Different parameters are discussed regarding their influence on (1) size and shape of the nanoparticles, (2) Cu/In ratio within the synthesised nanoparticles and (3) yield of the synthesis. Results show a strong dependency of the Cu/In ratio of the nanoparticles and the yield of the synthesis on the synthesis parameters. The influence of different parameters like (a) the ratio of metal cations to BH anions, (b) the Cu/In cation ratio within the precursor solution and (c) the dropping rate of the copper-indium precursor solution are discussed. The Cu/In ratio within the nanoparticles can mainly be controlled by the Cu/In cation ratio and the dropping rate of the copper-indium precursor solution. The yield of the synthesis shows saturation behaviour depending on the ratio of metal cations to BH anions. Shape and size of the nanoparticles are independent of the varied parameters. © 2013 Materials Research Societ
Keywords: nanoscale; alloy; chemical synthesis

Optimization of growth parameters for growth of high quality heteroepitaxial 3C-SiC films at 1200 degrees C - Wilhelm, Martin; Rieth, Marcel; Brandl, Marco; Wibowo, Rachmat Adhi; Hock, Rainer; Wellmann, Peter (Thin Solid Films, 2015)
In order to reduce the residual stress caused by the mismatch of thermal expansion coefficients of 3C-SiC layers grown on Si after cooling down to room temperature, the growth temperature was reduced from usually above 1300 degrees C to 1200 degrees C. Epitaxial layers with high crystalline quality were grown on 1 x 1 cm(2) (100) Si substrates. The layers were evaluated by means of x-ray diffraction (XRD), Raman measurements, scanning electron microscopy and atomic force microscopy. Full width at half maximum values of 0.19 degrees for XRD rocking curve measurements of the (200) 3C-SiC peak were achieved, indicating high crystalline quality of the layers and epitaxial growth. For optimized growth at 1200 degrees C a high C/Si-ratio, on-axis substrates and a layer thickness of over 1 mu m are necessary. (C) 2015 Elsevier B.V. All rights reserved.
Keywords: Cubic silicon carbide; Heteroepitaxy; X-ray diffraction; Raman spectroscopy

Optimization of KOH etching parameters for quantitative defect recognition in n- and p-type doped SiC - Sakwe, Aloysius; Müller, Ralf; Wellmann, Peter (Journal of Crystal Growth, 2006)
We have developed a KOH-based defect etching procedure for silicon carbide (SiC), which comprises in situ temperature measurement and control of melt composition. As benefit for the first time reproducible etching conditions were established (calibration plot, etching rate versus temperature and time); the etching procedure is time independent, i.e. no altering in KOH melt composition takes place, and absolute melt temperature values can be set. The paper describes this advanced KOH etching furnace, including the development of a new temperature sensor resistant to molten KOH. We present updated, absolute KOH etching parameters of n-type SiC and new absolute KOH etching parameters for low and highly p-type doped SiC, which are used for quantitative defect analysis. As best defect etching recipes we found T=530°C/ 5 min (activation energy: 16.4 kcal/mol) and T=500°C/ 5 min (activation energy: 13.5 kcal/mol) for n-type and p-type SiC, respectivel
Keywords: Defect selective etching; Dislocations; Etching; Temperature sensor; SiC

Optimization of processing solvent and film morphology to achieve efficient non-fullerene polymer solar cells processed in air - An, Kang; Zhong, Wenkai; Ying, Lei; Zhu, Peng; Fan, Baobing; Li, Zhenye; Li, Ning; Huang, Fei; Cao, Yong (Journal of Materials Chemistry C, 2020)
Polymer solar cells (PSCs) with remarkable power conversion efficiency and processability have been widely reported; however, most devices are processed using halogenated solvents under an inert atmosphere and require post-treatment to achieve optimal film morphology. In this manuscript, we developed efficient PSCs by combining a wide-bandgap conjugated polymer P2F-EHp and non-fullerene acceptors of IT-4F and IT-4Cl, which can be processed with non-chlorinated toluene:o-xylene co-solvent. It is interesting to note that a device based on IT-4Cl presented impressive photovoltaic performance with a power conversion efficiency of about 12%, which does not require post-treatment of solvent vapor annealing. The detailed investigation of film morphology by grazing incidence X-ray scattering and resonant soft X-ray scattering demonstrated that the co-solvent appeared to assist the manipulation of crystal coherent lengths and effectively decrease the phase separation of the corresponding blend films. Of particular importance is that this material system is compatible with the low-cost blade-coating technique using toluene:o-xylene co-solvent and can be processed under ambient conditions without post-treatment. A remarkable power conversion efficiency of 10.1% was achieved by blade-coating the P2F-EHp:IT-4F:IT-4Cl in air, which is slightly higher than that of 9.94% obtained from the spin-coating device processed in nitrogen. The results indicated that this material system is a promising candidate for constructing efficient PSCs toward practical applications.
CRIS-Team WoS Importer:2020-01-31

Optimization of Solution-Processed Luminescent Down-Shifting Layers for Photovoltaics by Customizing Organic Dye Based Thick Films - Solodovnyk, Anastasiia; Kick, Christopher; Osvet, Andres; Egelhaaf, Hans Joachim; Stern, Edda; Batentschuk, Miroslaw; Forberich, Karen; Brabec, Christoph (Energy Technology, 2016)
We theoretically and experimentally determined the optimized film properties of luminescent down-shifting (LDS) layers for thin film solar cells. Based on the predictions from an adapted optical model, we coated thick (300–500 mm) and efficient luminescent down-shifting layers from environmentally friendly solvents and industrially scalable inks. LDS layers consisted of polyvinyl butyral (PVB) as binder and organic luminescent dyes as UV-converters. The luminescence quantum yields of the dyes were studied in solution (benzyl alcohol) and for solid thick films. Our data shows that the studied dyes retain luminescent efficiencies of approximately 90% in the solid state when processed from solution. We further apply the produced layers onto copper indium gallium diselenide (CIGS) solar cells to verify the theoretical predictions for enhancing the external quantum efficiency (EQE) in the UV region. For the best converters a remarkable enhancement of the EQE from 9% to 52% was recorded at 380 nm. These findings underline that printed LDS layers indeed have the potential to enhance the efficiency and the light harvesting capabilities of industrially relevant photovoltaic modules.

Optimization of synthesis and compositional parameters of magnesium germanate and fluoro-germanate thermographic phosphors - Ali, Amjad; Khanzada, Laraib Sarfraz; Hashemi, Amir; Polzer, Carsten; Osvet, Andres; Brabec, Christoph; Batentschuk, Miroslaw (Journal of Alloys and Compounds, 2018)
Various documented formulations of magnesium germanate and flouro-germanate were synthesized by conventional solid state method at various temperatures to find out optimal synthesis conditions for maximum luminescence intensity. Relative efficiency of luminescence intensity and thermal stability of luminescence of these formulations is then discussed with respect to the commercially available phosphor. Mg28Ge7.5O38F10:Mn4+ was found to be the most efficient when synthesized at 1150 °C in air and has a comparable efficiency with respect to commercial sample. Photoluminescence intensity with increasing temperature was measured till 500 °C; the samples showed persistent emission till 300 °C. Relatively, samples with MgF2 give stronger emission at high temperatures if added in proper ratio.
Keywords: Mn4+ Red luminescence Magnesium fluoro-germanate Magnesium germanat Temperature dependent photoluminescence

Optimization of the SiC Powder Source Material for Improved Process Conditions During PVT Growth of SiC Boules - Ellefsen, Oda Marie; Arzig, Matthias; Steiner, Johannes; Wellmann, Peter; Runde, Pål (Materials, 2019)
We have studied the influence of different SiC powder size distributions and the sublimation behavior during physical vapor transport growth of SiC in a 75 mm and 100 mm crystal processing configuration. The evolution of the source material as well as of the crystal growth interface was carried out using in situ 3D X-ray computed tomography (75 mm crystals) and in situ 2D X-ray visualization (100 mm crystals). Beside the SiC powder size distribution, the source materials differed in the maximum packaging density and thermal properties. In this latter case of the highest packaging density, the in situ X-ray studies revealed an improved growth interface stability that enabled a much longer crystal growth process. During process time, the sublimation-recrystallization behavior showed a much smoother morphology change and slower materials consumption, as well as a much more stable shape of the growth interface than in the cases of the less dense SiC source. By adapting the size distribution of the SiC source material we achieved to significantly enhance stable growth conditions.
CRIS-Team Scopus Importer:2019-10-29
Keywords: Crystal growth; In situ visualization; SiC; Source material; Sublimation

Optimization of the SiC powder source size distribution for the sublimation growth of long crystal boules - Arzig, Matthias; Hsiao, TaChing; Wellmann, Peter (Advanced Materials Proceedings, 2018)
In this work we studied the influence of three different SiC powder size distributions and the sublimation behavior during physical vapor transport growth of SiC in a 3 inch crystal processing configuration. The evolution of the source material as well as of the crystal growth interface was carried out using in‐site X‐ray computed tomography. Two SiC powders exhibited a single modal D90 size distribution of ca. 50 μm and ca. 200 μm, respectively. In both cases the average SiC powder density was 1.2 g/cm3. The third powder was a mixture of the above mentioned source materials and exhibited a bimodal particle size distribution. The corresponding average SiC powder density was 1.7 g/cm3. In this latter case the in‐itu X‐ray computed tomography study revealed an improved growth interface stability that enabled a much longer crystal growth process. During process time, the sublimation‐recrystallization behavior of the mixed SiC powder showed a much smoother morphology change and slower materials consumption as well as much more stable shape of the growth interface than in the case of the less dense SiC source. By adapting the size distribution of the SiC source material we achieved to significantly enhance stable growth conditions.<br
Keywords: Silicon carbide; physical vapor transport; source material; in-situ visualization; computed tomography

Optimization of the SiC powder source size distribution for the sublimation growth of long crystals boules - Steiner, Johannes; Arzig, Matthias; Hsiao, Ta Ching; Wellmann, Peter (Materials Science Forum, 2019)
The influence of four different SiC source powder size distributions on the sublimation behavior during physical vapor transport growth of SiC was studied. The growth processes were carried out in a 3 inch crystal growth setup and observed in situ using advanced 3D computed tomography X-ray visualization. The single modal D90 size distribution of two source powders was 50 µm and 200 µm, respectively, with a corresponding average powder density of 1.17 g/cm³. The third source powder consisted of a blend of the previously named powders and exhibited an average powder density of 1.66 g/cm³ with a bimodal particle size distribution. The last source was composed of a solid polycrystalline SiC cylinder. The bimodal powder source exhibited a smoother morphology change and material consumption during the growth run and led to a much more stable shape change of the growth interface compared to the single modal source powders. The solid source featured the least morphology change. Therefore, with a careful adaption of the source material stable growth conditions can be achieved.
CRIS-Team Scopus Importer:2019-09-17
Keywords: Computed tomography; In-situ visualization; Physical vapor transport; Silicon carbide; Source material

Optimizing Microstructure Morphology and Reducing Electronic Losses in 1 cm2 Polymer Solar Cells to Achieve Efficiency over 15% - Fan, Baobing; Zeng, Zhaomiyi; Zhong, Wenkai; Ying, Lei; Zhang, Difei; Li, Meijing; Peng, Feng; Li, Ning; Huang, Fei; Cao, Yong (ACS Energy Letters, 2019)
The successful demonstration of high-performance organic solar cells (OSCs) on a relatively large area is vital for their industrial viability and future application. When the device area is enlarged from several mm2 to the scale of >1 cm2, critical losses caused by film inhomogeneity or defects in the photoactive layer strongly restrict the performance and reproducibility of OSCs. In this work, we demonstrate that through delicate optimization of a photoactive layer and minimization of optoelectronic losses, an impressive external quantum efficiency maximum up to 88% and an internal quantum efficiency peak of 97% are achieved for nonfullerene OSCs. Further incorporating fullerene as the third component into the photoactive layer optimizes the microstructure morphology, enabling large-scale devices with an area of >1.1 cm2, surpassing the 15% efficiency milestone. The exciting results demonstrated in this work highlight the strategic priority to minimize losses through both materials and electronic engineering toward high-performance large-area OSCs.
CRIS-Team Scopus Importer:2019-10-15

Optimizing Perovskite Thin-Film Parameter Spaces with Machine Learning-Guided Robotic Platform for High-Performance Perovskite Solar Cells - Zhang, Jiyun; Liu, Bowen; Liu, Ziyi; Wu, Jianchang; Arnold, Simon; Shi, Hongyang; Osterrieder, Tobias; Hauch, Jens; Wu, Zhenni; Luo, Junsheng; Wagner, Jerrit; Berger, Christian; et al. (Advanced Energy Materials, 2023)
Simultaneously optimizing the processing parameters of functional thin films remains a challenge. The design and utilization of a fully automated platform called SPINBOT is presented for the engineering of solution-processed functional thin films. The SPINBOT is capable of performing experiments with high sampling variability through the unsupervised processing of hundreds of substrates with exceptional experimental control. Through the iterative optimization process enabled by the Bayesian optimization (BO) algorithm, the SPINBOT explores an intricate parameter space, continuously improving the quality and reproducibility of the produced thin films. This machine learning (ML)-guided reliable SPINBOT platform enables the acceleration of the optimization process of perovskite solar cells via a simple photoluminescence characterization of films. As a result, this study arrives at an optimal film that, when processed into a solar cell in an ambient atmosphere, immediately yields a champion power conversion efficiency (PCE) of 21.6% with satisfactory performance reproducibility. The unsealed devices retain 90% of their initial efficiency after 1100 h of continuous operation at 60–65 °C under metal-halide lamps. It is anticipated that the integration of robotic platforms with the intelligent algorithm will facilitate the widespread adoption of effective autonomous experimentation to address the evolving needs and constraints within the materials science research community.
CRIS-Team Scopus Importer:2023-11-10
Keywords: closed-loop optimization; efficient and stable devices; machine learning; manufacturing optimization; perovskite thin films; PL characterization; robotic platform

Opto-electronics of PbS quantum dot and narrow bandgap polymer blends - Kahmann, Simon; Mura, Andrea; Protesescu, Loredana; Kovalenko, Maksym V.; Brabec, Christoph; Loi, Maria A. (Journal of Materials Chemistry C, 2015)
Here we report on the interaction between the narrow bandgap polymer [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta-[2,1-b;3,4-b]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and lead sulphide (PbS) colloidal quantum dots (CQDs) upon photoexcitation. We show that the presence of both materials in a blend leads to a significant reduction of photoluminescence (PL) lifetime of the polymer. This observation is attributed, supported by transient absorption (TA) data, to an efficient electron transfer towards the QDs for excitons generated on the polymer. Furthermore, the ligand capping the QD surface exhibits a great impact on the dynamics of the PL, with the long-chain oleic acid (OA) largely suppressing any kind of interaction. By means of external quantum efficiency (EQE) measurements we find evidence that both components give rise to a contribution to the photocurrent, making this an interesting blend for future applications in hybrid organic-inorganic solar cells.
Keywords: Engineering controlled terms: Copolymers; Energy gap; Polymer blends; Semiconductor quantum dots; Semiconductor quantum wells 2; 1; 3-Benzothiadiazole; Colloidal quantum dots; Electron transfer; External quantum efficiency; Future applications; Hybrid organic-inorganic; Narrow bandgap polymers; Transient absorption Engineering main heading: Quantum efficiency

Organic and perovskite solar modules innovated by adhesive top electrode and depth-resolved laser patterning - Spyropoulos, George D.; Quiroz, Cesar Omar Ramirez; Salvador, Michael Filipe; Hou, Yi; Gasparini, Nicola; Schweizer, Peter; Adams, Jens; Kubis, Peter; Li, Ning; Spiecker, Erdmann; Ameri, Tayebeh; Egelhaaf, Hans-Joachim; et al. (Energy and Environmental Science, 2016)
We demonstrate an innovative solution-processing fabrication route for organic and perovskite solar modules via depth-selective laser patterning of an adhesive top electrode. This yields unprecedented power conversion efficiencies of up to 5.3% and 9.8%, respectively. We employ a PEDOT:PSS-Ag nanowire composite electrode and depth-resolved post-patterning through beforehand laminated devices using ultra-fast laser scribing. This process affords low-loss interconnects of consecutive solar cells while overcoming typical alignment constraints. Our strategy informs a highly simplified and universal approach for solar module fabrication that could be extended to other thin-film photovoltaic technologies.
Keywords: Indexed keywords Engineering controlled terms: Conducting polymers; Electrodes; Laminated composites; Organic lasers; Perovskite; Solar cells; Solar power generation Depth-resolved; Fabrication routes; Innovative solutions; Laser patterning; Photovoltaic technology; Power conversion efficiencies; Solar module; Universal approach Engineering main heading: Solar cell arrays GEOBASE Subject Index: efficiency measurement; electrode; film; innovation; laser method; perovskite; photovoltaic system

Organic photovoltaic modules with new world record efficiencies - Distler, Andreas; Brabec, Christoph; Egelhaaf, Hans-Joachim (Progress in Photovoltaics, 2020)
During the last years, the development of new active materials has led to constant improvement in the power conversion efficiency (PCE) of solution-processed organic photovoltaics (OPV) to nowadays record values above 17% on small lab cells. In this work, we show the developments and results of a successful upscaling of such highly efficient OPV systems to the module level on large areas, which yielded two new certified world record efficiencies, namely, 12.6% on a module area of 26 cm2 and 11.7% on a module area of 204 cm2. The decisive developments leading to this achievement include the optimization of the module layout as well as the high-resolution short-pulse (nanosecond) laser structuring processes involved in the manufacturing of such modules. By minimizing the inactive areas within the total module area that are used for interconnecting the individual solar cells of the module in series, geometric fill factors of over 95% have been achieved. A production yield of 100% working modules during the manufacturing of these modules and an extremely narrow distribution of the final PCE values underline the excellent process control and reproducibility of the results. The new developments and their implementation into the production process of the record OPV modules are described in detail, along with the challenges that arose during this development. Finally, dark lock-in thermography (DLIT), electroluminescence (EL), and photoluminescence (PL) measurements of the record module are presented.
CRIS-Team Scopus Importer:2020-09-25
Keywords: certified power conversion efficiency (PCE); laser patterning; organic photovoltaics (OPV); solar modules; world record

Organic photovoltaics for low light applications - Steim, Roland; Ameri, Tayebeh; Schilinsky, Pavel; Waldauf, Christoph; Dennler, Gilles; Scharber, Markus; Brabec, Christoph (Solar Energy Materials and Solar Cells, 2011)
Here we report on organic photovoltaics (OPV) suitable for low light applications. In this paper, we illustrate the impact of R and R for indoor and outdoor applications. In addition, we propose a simple physics approach to predict the behavior of organic solar cells under various illumination intensities through electrical modeling. The combination of simulation and modeling allows to define a set of design rules for OPVs under low light illumination. The performance of various organic solar cells under low light intensity is compared with our predictions and excellent correlation is found. OPV shows high performance under low light conditions. © 2011 Elsevier B.V. All rights reserved.
Keywords: Indoor; Low light; OPV; Organic photovoltaics; Shunt resistance; Simulation

Organic Photovoltaics: Materials, Device Physics, and Manufacturing Technologies: Second Edition - Brabec, Christoph; Scherf, Ullrich; Dyakonov, Vladimir V. (Wiley Blackwell, 2014)
The versatility of organic photovoltaics is already well known and this completely revised, updated, and enlarged edition of a classic provides an up-to-date overview of this hot topic. The proven structure of the successful first edition, divided into the three key aspects of successful device design: materials, device physics, and manufacturing technologies, has been retained. Important aspects such as printing technologies, substrates, and electrode systems are covered. The result is a balanced, comprehensive text on the fundamentals as well as the latest results in the area that will set R&D trends for years to come. With its combination of both academic and commercial technological views, this is an optimal source of information for scientists, engineers, and graduate students already actively working in this field, and looking for comprehensive summaries on specific topics.

Organic solar cells using oligomer acceptors for improved stability and efficiency - Liang, Youcai; Zhang, Difei; Wu, Zerun; Jia, Tao; Lüer, Larry; Tang, Haoran; Hong, Ling; Zhang, Jiabin; Zhang, Kai; Brabec, Christoph; Li, Ning; Huang, Fei (Nature Energy, 2022)
The power conversion efficiencies of organic solar cells (OSCs) have reached over 19%. However, the combination of high efficiency and long-term stability is still a major conundrum of commercialization. Here a Y6-analogue and a 2,2′-bithiophene unit are utilized to construct a series of oligomer acceptors to investigate the effect of molecular size and packing properties on photovoltaic performance. By altering the molecular chain length, we modify the thermal properties, crystallization behaviours and molecular packing and achieve an optimal microstructure and a more stable morphology in blend films. A combination of efficiencies over 15% and an extrapolated T80 lifetime over 25,000 h, which equates to an average lifetime exceeding 16 years in Guangzhou, is achieved for binary OSCs based on an oligomer acceptor. This work emphasizes the importance of oligomeric strategy in tuning molecular packing behaviours and blend morphology, leading to development of novel non-fullerene acceptors for stable and efficient OSCs.
CRIS-Team Scopus Importer:2022-11-25

Organic Tandem Solar Cells - Dennler, Gilles; Sariciftci, Niyazi Serdar; Brabec, Christoph; Annamalai, V.E. (Reference Module in Materials Science and Materials Engineering 2016, 2016)
The article traces the developments in the area of organic tandem solar cells. By piling several solar devices one over the other, a tandem cell is obtained. The light which is not absorbed in the lower cell can be absorbed in the upper cell. The thermalization losses are lowered due to the usage of materials having different bandgaps. By finding the appropriate dopants for both the p and the n region the efficiency of the stacked device can be boosted to 3.8%, which is 80% larger than that of a single device. It is postulated that efficiencies as high as 15% can be achieve
Keywords: Band gap Dopant Organic solar cell Polymeric solar cell Power conversion efficiency Single junction solar cell Solar device Solar tandem cell

Organic tandem solar cells - Li, Ning; Ameri, Tayebeh; Brabec, Christoph (CRC Press, 2015)

Organic ternary solar cells: A review - Ameri, Tayebeh; Khoram, Parisa; Min, Jie; Brabec, Christoph (Advanced Materials, 2013)
Recently, researchers have paid a great deal of attention to the research and development of organic solar cells, leading to a breakthrough of over 10% power conversion efficiency. Though impressive, further development is required to ensure a bright industrial future for organic photovoltaics. Relatively narrow spectral overlap of organic polymer absorption bands within the solar spectrum is one of the major limitations of organic solar cells. Among different strategies that are in progress to tackle this restriction, the novel concept of ternary organic solar cells is a promising candidate to extend the absorption spectra of large bandgap polymers to the near IR region and to enhance light harvesting in single bulk-heterojunction solar cells. In this contribution, we review the recent developments in organic ternary solar cell research based on various types of sensitizers. In addition, the aspects of miscibility, morphology complexity, charge transfer dynamics as well as carrier transport in ternary organic composites are addressed. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: dyes; IR sensitization; low bandgap polymers; organic solar cells; small molecules; ternary blends

Organometal halide perovskite imager: a comparison 1.5 years after fabrication - Deumel, Sarah; Van Breemen, Albert J.J.M.; Peeters, Bart; Maas, Joris; Akkerman, Hylke B.; Meulenkamp, Eric A.; Gelinck, Gerwin H.; Huerdler, Judith E.; Schmidt, Oliver; Heiß, Wolfgang; Tedde, Sandro F. (Progress in Biomedical Optics and Imaging - Proceedings of SPIE, 2022)
The next generation medical imaging will benefit significantly from artificial intelligence - therefore, not only advances in computing power are required, but also further materials and technology improvements will lead to better image quality. The strong X-ray absorption, high charge carrier mobility and lifetime recommend perovskites like methylammonium lead triiodide (MAPbI3) as novel direct X-ray converting materials. A major obstacle to the commercialization is the limited stability and lifetime, as reported until now. Here, we show for the first time that degradation is limited in our X-ray detector after 1.5 years of storage in ambient conditions.
CRIS-Team Scopus Importer:2022-06-10
Keywords: direct conversion; lifetime; medical imaging; perovskite; pixelated imaging detector; stability; X-Ray detector

Organometallic perovskites for optoelectronic applications (Conference Presentation) - Levchuk, Ievgen; Högl, Florian; Brandl, Marco; Osvet, Andres; Herre, Patrick; Hock, Rainer; Schweizer, Peter; Spiecker, Erdmann; Batentschuk, Miroslaw; Brabec, Christoph; Peukert, Wolfgang (SPIE Nanoscience + Engineering, 2016, 2016)
Organometallic halide perovskites CH3NH3BX3 (B= Pb, Sn, Ge; X = I, Br, Cl) have become one of the most promising semiconductors for solar cell applications, reaching power conversion efficiencies beyond 20%. Improving our ability to harness the full potential of organometal halide perovskites requires the development of more reliable synthesis routines of well defined, reproducible and defect free reference systems allowing to study the fundamental photo-physical processes. In this study we present size and band gap engineering for organo-lead perovskites crystallites with various shapes and sizes ranging from the 5 nm regime all the way to 1 cm. Colloidal nano-crystals, micro-crystlline particles as well as single crystals are demonstrated with excellent purity and control in shape and size are demonstrated. The structural, optical and photo-physical properties of these reference materials are investigated and analyzed as function of their size and shape.
Keywords: particle; crystal; organometal; optoelectronics

Outdoor photoluminescence of large area photovoltaic modules - Doll, Bernd; Hepp, Johannes; Hoffmann, Mathis; Talkenberg, Florian; Schüler, Renè; Baier, Manuel; Buerhop-Lutz, Claudia; Tegtmeyer, Dirk; Peters, Ian Marius; Hauch, Jens; Brabec, Christoph (Proceedings Volume 11809, Organic, Hybrid, and Perovskite Photovoltaics XXII, 2021)
Cost-effective, fast, and non-destructive, on-site photovoltaic (PV) characterization methods are of interest to PV operators to determine countermeasures against defects causing power loss or against safety problems. Combining the advantages of both methods electroluminescence (EL) and thermography is photoluminescence (PL). With our PL setup, we achieved high resolution luminescence images of large area PV modules without any physical and electrical contact. Defects are recognizable with a high rate compared to indoor EL images at controlled conditions. We analyzed inactive areas, cracks, potential induced degradation, snail trails, EVA degradation, and interconnection failures and compared the PL images with different characterization methods.
Keywords: Luminescene; Solar cells; Electroluminescence; Photovoltaics; Image resolution; Light sources; Safety; Sensors; Silicon; Thermography

Overcoming efficiency and cost barriers for large-area quantum dot photovoltaics through stable ink engineering - Shi, Guozheng; Ding, Xiaobo; Liu, Zeke; Liu, Yang; Chen, Yifan; Liu, Cheng; Ni, Zitao; Wang, Haibin; Ito, Katsuji; Igarashi, Keisuke; Feng, Kun; Zhang, Kaicheng; et al. (Nature Energy, 2025)
The bottom-up construction of electronics from colloidal quantum dots (CQDs) could innovate nanotechnology manufacturing through printing. However, the unstable and expensive semiconductive CQD inks make the scaling up of CQD electronics challenging. Here we develop a strategy for engineering the solution chemistry of lead sulfide (PbS) CQD inks prepared from a low-cost direct synthesis method. By creating an iodine-rich environment in weakly coordinating solvents, we convert the iodoplumbates into functional anions, which condense into a robust surface shell. The fully charged electrostatic surface layer prevents aggregation and epitaxial fusion of CQDs, yielding stable inks. By eliminating the fusion-induced inter-band states, we print a compact CQD film with uniformity in three dimensions, flattened energy landscape and improved carrier transport. We achieved a certified efficiency of 13.40% on 0.04 cm2 cells, with a 300-fold increase in active area, scaling up to a 12.60 cm2 module with a certified efficiency of 10%.
CRIS-Team Scopus Importer:2025-04-18

Overcoming efficiency and stability limits in water-processing nanoparticular organic photovoltaics by minimizing microstructure defects - Xie, Chen; Heumüller, Thomas; Gruber, Wolfgang; Tang, Xiaofeng; Classen, Andrej; Schuldes, Isabel; Bidwell, Matthew; Späth, Andreas; Fink, Rainer; Unruh, Tobias; Mcculloch, Iain; Li, Ning; et al. (Nature Communications, 2018)
There is a strong market driven need for processing organic photovoltaics from eco-friendly solvents. Water-dispersed organic semiconducting nanoparticles (NPs) satisfy these premises convincingly. However, the necessity of surfactants, which are inevitable for stabilizing NPs, is a major obstacle towards realizing competitive power conversion efficiencies for water-processed devices. Here, we report on a concept for minimizing the adverse impact of surfactants on solar cell performance. A poloxamer facilitates the purification of organic semiconducting NPs through stripping excess surfactants from aqueous dispersion. The use of surfactant-stripped NPs based on poly(3-hexylthiophene) / non-fullerene acceptor leads to a device efficiency and stability comparable to the one from devices processed by halogenated solvents. A record efficiency of 7.5% is achieved for NP devices based on a low-band gap polymer system. This elegant approach opens an avenue that future organic photovoltaics processing may be indeed based on non-toxic water-based nanoparticle inks.
Keywords: microstructure-recombination correlation; organic photovoltaics; polymer-fullerene nanoparticles; water processed

Overcoming Electrode-Induced Losses in Organic Solar Cells by Tailoring a Quasi-Ohmic Contact to Fullerenes via Solution-Processed Alkali Hydroxide Layers - Zhang, Hong; Shallcross, R. Clayton; Li, Ning; Stubhan, Tobias; Hou, Yi; Chen, Wei; Ameri, Tayebeh; Turbiez, Mathieu; Armstrong, Neal R.; Brabec, Christoph (Advanced Energy Materials, 2016)
It is shown that the performance of inverted organic solar cells can be significantly improved by facilitating the formation of a quasi-ohmic contact via solution-processed alkali hydroxide (AOH) interlayers on top of n-type metal oxide (aluminum zinc oxide, AZO, and zinc oxide, ZnO) layers. AOHs significantly reduce the work function of metal oxides, and are further proven to effectively passivate defect states in these metal oxides. The interfacial energetics of these electron collecting contacts with a prototypical electron acceptor (C) are investigated to reveal the presence of a large interface dipole and a new interface state between the Fermi energy and the C highest occupied molecular orbital for AOH-modified AZO contacts. These novel interfacial gap states are a result of ground-state electron transfer from the metal hydroxide-functionalized AZO contact to the adsorbed molecules, which are hypothesized to be electronically hybridized with the contact. These interface states tail all the way to the Fermi energy, providing for a highly n-doped (metal-like) interfacial molecular layer. Furthermore, the strong "light-soaking" effect is no longer observed in devices with a AOH interface. Solution-processed alkali hydroxides significantly reduce the work function of metal oxides, such as zinc oxide or aluminum zinc oxide (AZO), and are further proven to effectively passivate defect states in these metal oxides. The interface states with alkali hydroxide-modified AZO contacts tail all the way to the Fermi energy, providing for a highly n-doped (metal-like) interfacial molecular layer.
Keywords: alkali hydroxide; interfacial layers; organic solar cells; solution processing

Overcoming incompatibility of donors and acceptors by constructing planar heterojunction organic solar cells - Zhang, Difei; Zhong, Wenkai; Ying, Lei; Fan, Baobing; Li, Meijing; Gan, Ziqi; Zeng, Zhaomiyi; Chen, Dongcheng; Li, Ning; Huang, Fei; Cao, Yong (Nano Energy, 2021)
Nanoscale phase separation plays a critical role in achieving high photovoltaic performance in bulk-heterojunction organic solar cells, which is limited by solubility, crystallinity, and the compatibility of the donors and acceptors. However, these limits can be overcome by new applications of the traditional planar heterojunction structure. Herein, we demonstrate quasi-orthogonal solvents for a wide-band-gap donor (P2F-EHp) and a highly crystalline non-fullerene acceptor (M4–4F) to overcome the over-agglomeration and enabled the fabrication of a planar heterojunction device with a favorable power conversion efficiency of 14.2% (compared with 12.7% in bulk-heterojunction device), which is among one of the highest efficiencies of planar heterojunction devices to date. Detailed studies based on cross-sectional transmittance electron microscopy, grazing incidence wide-angle X-ray scattering at various incident angles, and resonance soft X-ray scattering measurements confirmed the formation of optimal nanoscale morphology. Further transient absorption measurements also proved that the exciton dissociation and free-carrier generation efficiency are not limited by the small mixed-phase area and large pure domains. These findings show insights into the intrinsic morphological changes and charge transfer process in bulk-heterojunction and planar heterojunction structures, providing a novel method for matching donors and acceptors with relatively poor compatibility.
CRIS-Team Scopus Importer:2021-03-19
Keywords: Film morphology; Layer-by-layer processing; Organic solar cells; Planar heterojunction structure

Overcoming interface losses in organic solar cells by applying low temperature, solution processed aluminum-doped zinc oxide electron extraction layers - Stubhan, Tobias; Litzov, Ivan; Li, Ning; Salinas Batallas, Michael; Steidl, Matthias; Sauer, Gerhard; Forberich, Karen; Matt, Gebhard; Halik, Marcus; Brabec, Christoph (Journal of Materials Chemistry A, 2013)
Intrinsic zinc oxide (ZnO) is widely used as an electron extraction layer (EEL) for inverted polymer solar cells. Despite the excellent device performance, a major drawback for large area production is its low conductivity. Using microscopic simulations, we derived a technically reasonable threshold value of 10−3 S cm−1 for the conductivity required to overcome transport limitations. For conductivity values typical for ZnO we observed the interface layer thickness restriction at only a few tens of nanometers, either as a fill factor drop due to serial resistance, eventually accompanied by a second diode behavior, or by the need for light soaking. Higher conductive aluminum-doped zinc oxide (AZO), which was introduced earlier, meets the desired conductivity threshold, however, at the cost of high temperature processing. High annealing temperatures (>150 °C) significantly improve the electrical properties of ZnO, but prohibit processing on plastic substrates or organic active layers. Here we report on AZO layers from a sol–gel precursor, which has been already reported to give sufficiently high conductivities at lower processing temperatures (<150 °C). We investigate the influence of different precursor compositions on the electrical properties of the thin films and their performance in inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. Low temperature AZO layers with thicknesses up to 680 nm maintained comparable performance to devices with thin AZO layers.
Keywords: Aluminum-doped zinc oxide; Annealing temperatures; High-temperature processing; Inverted polymer solar cells; Microscopic simulation; Organic active layers; Poly (3-hexylthiophene); Processing temperature Engineering controlled terms: Aluminum; Electric properties; Extraction; Processing; Sol-gels; Solar cells; Temperature Engineering main heading: Zinc oxide

Overcoming Interfacial Losses in Solution-Processed Organic Multi-Junction Solar Cells - Du, Xiaoyan; Lytken, Ole; Killian, Manuela; Cao, Jiamin; Stubhan, Tobias; Turbiez, Mathieu; Schmuki, Patrik; Steinrück, Hans-Peter; Ding, Liming; Fink, Rainer; Li, Ning; Brabec, Christoph (Advanced Energy Materials, 2016)
Organic solar cells are promising in terms of full-solution-processing which enables low-cost and large-scale fabrication. While single-junction solar cells have seen a boost in power conversion efficiency (PCE), multi-junction solar cells are promising to further enhance the PCE. In all-solution-processed multi-junction solar cells, interfacial losses are often encountered between hole-transporting layer (HTL) and the active layers and therefore greatly limit the application of newly developed high-performance donor and acceptor materials in multi-junction solar cells. Here, the authors report on a systematic study of interface losses in both single-junction and multi-junction solar cells based on representative polymer donors and HTLs using electron spectroscopy and time-of-flight secondary ion mass spectrometry. It is found that a facile mixed HTL containing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and MoO nanoparticles successfully overcomes the interfacial losses in both single- and multi-junction solar cells based on various active layers by reducing interface protonation, promoting better energy-level alignment, and forming a dense and smooth layer. Solution-processed single-junction solar cells are demonstrated to reach the same performance as with evaporated MoO (over 7%). Multi-junction solar cells with polymers containing nitrogen atoms as the first layer and the mixed PEDOT:PSS and MoO nanoparticles as hole extraction layer reach fill factor (FF) of over 60%, and PCE of over 8%, while the identical stack with pristine PEDOT:PSS or MoO nanoparticles show FF smaller than 50% and PCE less than 5%.
Keywords: Interfacial losses; Organic multi-junction solar cells; Organic photovoltaics; Recombination layer; Solution-processing

Overcoming Microstructural Limitations in Water Processed Organic Solar Cells by Engineering Customized Nanoparticulate Inks - Xie, Chen; Classen, Andrej; Späth, Andreas; Tang, Xiaofeng; Min, Jie; Meyer, Markus; Zhang, Chaohong; Li, Ning; Osvet, Andres; Fink, Rainer; Brabec, Christoph (Advanced Energy Materials, 2018)
The application of conjugated polymer and fullerene water-based nanoparticles (NP) as ecofriendly inks for organic photovoltaics (OPVs) is reported. A low bandgap polymer diketopyrrolopyrrole–quinquethiophene (PDPP5T-2) and the methanofullerene PC71BM are processed into three types of nanoparticles: pristine fullerene NPs, pristine polymer NPs, and mixed polymer:fullerene NPs, allowing the formation of bulk heterojunction (BHJ) composites with different domain sizes. Mild thermal annealing is required to melt the nanospheres and enable the formation of interconnected pathways within mixed phases. This BHJ is accompanied by a shrinkage of film, whereas the more compact layers show enhanced mobility. Consistently reduced recombination and better performance are found for mixed NP, containing both, the polymer and the fullerene within a single NP. The optimized solar cell processed by ultrasmall NPs delivers a power conversion efficiency of about 3.4%. This is among the highest values reported for aqueous processed OPVs but still lacks performance compared to those being processed from halogenated solvents. Incomplete crystallization is identified as the main root for reduced efficiency. It is nevertheless believed that postprocessing does not cut attraction from printing aqueous organic NP inks as a trendsetting strategy for the reliable and ecofriendly production of organic solar cells.
Keywords: Microstructure-recombination correlation; Organic photovoltaics; Polymer-fullerene nanoparticles; Water processed

Overcoming Moisture-Induced Degradation in Organic Solar Cells - Wachsmuth, Josua; Distler, Andreas; Deribew, Dargie; Salvador, Michael Filipe; Brabec, Christoph; Egelhaaf, Hans-Joachim (Advanced Engineering Materials, 2023)
Unencapsulated organic solar cells are prone to severe performance losses in the presence of moisture. Accelerated damp heat (85 °C/85% RH) studies are presented and it is shown that the hygroscopic hole-transporting PEDOT:PSS layer is the origin of device failure in the case of prototypical inverted solar cells. Complementary measurements unveil that under these conditions a decreased PEDOT:PSS work function along with areas of reduced electrical contact between active layer and hole-transport layer are the main factors for device degradation rather than a chemical reaction of water with the active layer. Replacements for PEDOT:PSS are explored and it is found that tungsten oxide (WO3) or phosphomolybdic acid (PMA)—materials that can be processed from benign solvents at room temperature—yields comparable performance as PEDOT:PSS and enhances the resilience of solar cells under damp heat. The stability trend follows the order PEDOT:PSS << WO3 < PMA, with PEDOT:PSS-based devices failing after few minutes, while PMA-based devices remain nearly pristine over several hours. PMA is thus proposed as a robust, solution-processable hole extraction layer that can act as a one to one replacement of PEDOT:PSS to achieve organic solar cells with significantly improved longevity.
CRIS-Team Scopus Importer:2023-06-02
Keywords: degradation and stability; humidity; organic photovoltaics; phosphomolybdic acid; solution processed

Overcoming optical losses in thin metal-based recombination layers for efficient n-i-p perovskite-organic tandem solar cells - Tian, Jingjing; Liu, Chao; Forberich, Karen; Barabash, Anastasia; Xie, Zhiqiang; Qiu, Shudi; Byun, Jiwon; Peng, Zijian; Zhang, Kaicheng; Du, Tian; Sathasivam, Sanjayan; Macdonald, Thomas J.; et al. (Nature Communications, 2025)
Perovskite-organic tandem solar cells (P-O-TSCs) hold substantial potential to surpass the theoretical efficiency limits of single-junction solar cells. However, their performance is hampered by non-ideal interconnection layers (ICLs). Especially in n-i-p configurations, the incorporation of metal nanoparticles negatively introduces serious parasitic absorption, which alleviates photon utilization in organic rear cell and decisively constrains the maximum photocurrent matching with front cell. Here, we demonstrate an efficient strategy to mitigate optical losses in Au-embedded ICLs by tailoring the shape and size distribution of Au nanoparticles via manipulating the underlying surface property. Achieving fewer, smaller, and more uniformly spherical Au nanoparticles significantly minimizes localized surface plasmon resonance absorption, while maintaining efficient electron-hole recombination within ICLs. Consequently, optimized P-O-TSCs combining CsPbI2Br with various organic cells benefit from a substantial current gain of >1.5 mA/cm2 in organic rear cells, achieving a champion efficiency of 25.34%. Meanwhile, optimized ICLs contribute to improved long-term device stability.
CRIS-Team Scopus Importer:2025-01-31

Overcoming photovoltage deficitvianatural amino acid passivation for efficient perovskite solar cells and modules - Hu, Jinlong; Xu, Xin; Chen, Yijun; Wu, Shaohang; Wang, Zhen; Wang, Yousheng; Jiang, Xiaofang; Cai, Boyuan; Shi, Tingting; Brabec, Christoph; Mai, Yaohua; Guo, Fei (Journal of Materials Chemistry A, 2021)
Electronic defects at grain boundaries and surfaces of perovskite crystals impair the photovoltaic performance and stability of solar devices. In this work, we report the compensation of photovoltage losses in blade-coated methylammonium lead triiodide (MAPbI3) devicesviapassivation with natural amino acid (NAA) molecules. We found that the optoelectronic properties of NAA-passivated perovskite films and the corresponding device performances are closely correlated with the molecular interaction strength. A side-by-side comparative study of four typical NAAs reveals that arginine (Arg) functionalized with a guanidine end group exhibits optimum passivation effects owing to the strongest coordinative bonding with the uncoordinated Pb2+, which markedly suppresses the detrimental antisite PbIdeep level defects. As a result, nonradiative charge recombination is significantly reduced, resulting in a substantially increased open-circuit voltage (VOC) of 1.17 V and a high efficiency of 20.49%. A solar module with an active area of 10.08 cm2is also fabricated, yielding an efficiency of 15.65% with negligibleVOClosses. In parallel, the Arg-passivated solar devices exhibit enhanced operational stability due to the formation of a hydrophilic Arg protective layer which encapsulates the perovskite crystals.
CRIS-Team Scopus Importer:2021-03-19

Overcoming Temperature-Induced Degradation of Silver Nanowire Electrodes by an Ag@SnOx Core-Shell Approach - Kalancha, Violetta; These, Albert; Vogl, Lilian; Levchuk, Ievgen; Zhou, Xin; Barr, Maissa; Bruns, Mark; Bachmann, Julien; Virtanen, Sannakaisa; Spiecker, Erdmann; Osvet, Andres; Brabec, Christoph; et al. (Advanced Electronic Materials, 2022)
Transparent electrodes consisting of silver nanowires (Ag NWs) are a solution-processed alternative to commonly used indium tin oxide electrodes. Here, Ag NW electrodes protected by a tin oxide (SnOx) are explored and unprecedented thermal stability is found. While unprotected Ag NW electrodes fail at 250 degrees C, the SnOx Ag NW electrodes remain stable for 40 h at 250 degrees C and withstand high temperatures up to 500 degrees C for short times. First, an optimized method of synthesis that provides uniform Ag NWs with high reproducibility is used. Afterward, a SnOx shell is formed in a wet chemical reaction. Fabrication of highly conductive electrodes requires thermal annealing at 300 degrees C for 5 min under ambient atmosphere. Electrodes with a sheet resistance as low as 20 ohm sq(-1) and visible transmittance of 84% are demonstrated. It is shown that a approximate to 2 nm thick SnOx shell effectively protects the Ag NWs in a temperature range between 200 and 500 degrees C, whereas unprotected Ag NWs suddenly fail at temperatures beyond 200 degrees C. It is strongly anticipated that these improvements in the stability of Ag NWs open a large field of further investigations and applications.
CRIS-Team WoS Importer:2022-02-11

Overcoming the Interface Losses in Planar Heterojunction Perovskite-Based Solar Cells - Hou, Yi; Chen, Wei; Baran, Derya; Stubhan, Tobias; Luechinger, Norman A.; Hartmeier, Benjamin; Richter, Moses; Min, Jie; Chen, Shi; Ramírez Quiroz, César Omar; Li, Ning; Zhang, Hong; et al. (Advanced Materials, 2016)
An experiment was conducted to demonstrate that low-temperature processed NiO-based nanocrystal ink (LT-NiO) has the potential to form an almost loss-free hole selective interface for flat heterojunction perovskite-based solar cells. First, The patterned ITO substrates were ultrasonic cleaned with acetone and isopropanol for 10 min each. On cleaned ITO substrate, a dense and smooth layer of LT-NiO was deposited by spin coating and followed by annealing at 70?230°C for 10 min in air to remove organic components. The as-prepared perovskite precursor solution was filtered using 0.45 μm PTFE syringe filter and coated onto the ITO/LT-NiO substrate at a speed of 4000 r.p.m. for 35 s. J-V characteristics of all the devices were measured using a source measurement unit from BoTest. FTPS:FTPS-EQE measurements were carried out using a Vertex 70 from Brucker optics, equipped with QTH lamp, quartz beam splitter and external detector option. A major improvement in open circuit voltage is found by replacing PEDOT:PSS with LT-NiO. A detailed analysis reveals that LT-NiO significantly reduces non-radiative recombination at the PEDOT:PSS/perovskite interface and further enhances the radiative LED efficiency towards unity which brings open circuit voltage closer to the radiative limit.
Keywords: low temperature processing; nickel oxide; non-radiative voltage loss; perovskite solar cells

Overcoming the morphological and efficiency limit in all-polymer solar cells by designing conjugated random copolymers containing a naphtho[1,2c:5,6-c']bis([1,2,5] thiadiazole)] moiety - Xie, Ruihao; Li, Zhenye; Zhong, Wenkai; Ying, Lei; Hu, Qin; Liu, Feng; Li, Ning; Russell, Thomas P.; Huang, Fei; Cao, Yong (Journal of Materials Chemistry A, 2018)
The ternary concept based on two electron donors and one electron acceptor has been deployed to extend the absorption as well as photovoltaic performance of all-polymer solar cells (all-PSCs). However, it is still challenging to achieve the delicate morphology of ternary blended bulk-heterojunction films through rational molecular design. To overcome these issues, herein, we designed and synthesized a series of narrow bandgap conjugated copolymers by combining two conjugated polymers with various molar ratios of the naphthobis[1,2,5]thiadiazole (NT) unit (namely, NT00-NT100). All-PSCs were constructed by integrating these resultant copolymers with a commercially available electron-accepting copolymer N2200. The resultant all-PSCs based on the ternary copolymers NT40 exhibited an impressively high power conversion efficiency over 8.0%, which was clearly superior to that obtained for devices based on the NT00:NT100:N2200 blended films. Further characterization of the morphology of the films revealed that the incorporation of the NT unit improved - stacking and a degree of phase separation, which are conducive to exciton diffusion and charge transport.

Overcoming the Thermal Instability of Efficient Polymer Solar Cells by Employing Novel Fullerene-Based Acceptors - Zhang, Chaohong; Mumyatov, Alexander; Langner, Stefan; Perea Ospina, Jose Dario; Kassar, Thaer; Min, Jie; Ke, Lili; Chen, Haiwei; Gerasimov, Kirill L.; Anokhin, Denis V.; Ivanov, Dimitri A.; Ameri, Tayebeh; et al. (Advanced Energy Materials, 2017)
Solution-processed organic solar cells with promising photovoltaic performance and extraordinary high thermal stability are achieved by employing novel fullerene-based acceptors in combination with two state-of-the-art polymer donors. The findings demonstrated in this work underline the necessity and importance of novel acceptor design rules for highly efficient organic solar cells with excellent device stability.
Keywords: Miscibility; Morphological analysis; Novel fullerene-based acceptors; Organic photovoltaics; Thermal stability

Overgrowth of Protrusion Defects during Sublimation Growth of Cubic Silicon Carbide Using Free-Standing Cubic Silicon Carbide Substrates - Schöler, Michael; La Via, Francesco; Mauceri, Marco; Wellmann, Peter (Crystal Growth and Design, 2021)
We investigated the overgrowth of protrusion defects during sublimation growth of cubic silicon carbide (3C-SiC) using free-standing on-axis and off-axis substrates. Three different overgrowth mechanisms were found to contribute to defect elimination: (i) mutual overgrowth by defects of the same type, (ii) real overgrowth by step-flow growth, and (iii) overgrowth by quasi-step-flow growth at surface irregularities. Mechanisms (i) and (iii) are not real overgrowth mechanisms because they are directly linked to the inducing defects themselves or other undesired disturbances of the substrate. However, at high defect densities or for the on-axis substrate, they represent a relevant elimination mechanism. Overgrowth according to (ii) is possible only for off-axis substrates and represents a real elimination mechanism, leading to an improvement of the material quality. In the context of this work, we provide a phenomenological description of the overgrowth principle as well as limitations for the defect elimination, depending on the structure of the protrusions. For the first time, the overgrowth of protrusions has been categorized for different types and qualities of substrates. Fundamental differences were identified, which can lead to a more focused further development of defect elimination in 3C-SiC.
CRIS-Team Scopus Importer:2021-07-02

Over one-micron-thick void-free perovskite layers enable highly efficient and fully printed solar cells - Qiu, Shudi; Majewski, Martin; Dong, Lirong; Distler, Andreas; Li, Chaohui; Forberich, Karen; Tian, Jingjing; Hemasiri, Naveen Harindu; Liu, Chao; Zhang, Jiyun; Yang, Fu; Le Corre, Vincent Marc; et al. (Energy and Environmental Science, 2025)
Commercialization of perovskite photovoltaics hinges on the successful transition from laboratory-scale fabrication to industrial-scale manufacturing. A key challenge in fully printed perovskite solar cells with non-reflecting back electrodes is the deposition of high-quality, over-one-micron-thick perovskite layers to minimize photocurrent losses from incomplete light absorption. However, the formation of voids at the substrate/perovskite interface impedes the fabrication of such layers. Here, phase-field simulations reveal that the bottom voids originate from trapped residual solvents, driven by nanocrystal aggregation at the liquid-vapor interface during drying. Guided by these insights, we introduce a two-dimensional (2D) perovskite layer-assisted growth strategy to promote heterogeneous nucleation at the substrate, accelerating 3D perovskite crystallization and preventing solvent entrapment. This strategy enables the formation of highly crystalline, monolithic perovskite films exceeding one micrometer in thickness. The resulting void-free films maximize photocurrent extraction, achieving power conversion efficiencies of 19.9% on rigid substrates and 17.5% on flexible substrates in fully printed perovskite solar cells with non-reflecting carbon electrodes.
CRIS-Team Scopus Importer:2025-05-30

P3HT: Non-fullerene acceptor based large area, semi-transparent PV modules with power conversion efficiencies of 5%, processed by industrially scalable methods - Strohm, S.; Machui, F.; Langner, Stefan; Kubis, P.; Gasparini, Nicola; Salvador, Michael Filipe; Mcculloch, I.; Egelhaaf, H. -J.; Brabec, Christoph (Energy and Environmental Science, 2018)

Panchromatic ternary/quaternary polymer/fullerene BHJ solar cells based on novel silicon naphthalocyanine and silicon phthalocyanine dye sensitizers - Ke, Lili; Gasparini, Nicola; Min, Jie; Zhang, Hong; Adam, Matthias; Rechberger, Stefanie; Forberich, Karen; Zhang, Chaohong; Spiecker, Erdmann; Tykwinski, Rik; Brabec, Christoph; Ameri, Tayebeh (Journal of Materials Chemistry A, 2017)
More efficient light harvesting throughout the whole solar spectrum by introducing third and fourth components offers a new pathway towards the development of high efficiency organic solar cells based on polymer/fullerene blends. Recently, dye molecules have been utilized as promising light harvesting photosensitizers in the near-IR region. Herein, we report the design, synthesis and application of a novel silicon naphthalocyanine (SiNC-1) as an efficient photosensitizer in single dye ternary devices as well as in multi-colored co-sensitized quaternary devices, incorporating a silicon phthalocyanine (SiPC-0 or SiPC-1) as the fourth component to complement the spectral absorption of the SiNC-1. The dominant complex charge transfer/transport mechanism behind the enhanced photosensitivity of the ternary blend has been investigated by means of electrical, optical, and advanced characterization techniques. External quantum efficiency (EQE) measurements on multi-colored dye sensitized devices covering the UV-vis as well as near-IR regions from 350 up to 900 nm outline apparent signal characteristics of each single dye, corroborating the effective contribution of both SiPC and SiNC dyes to enhancing the short-circuit current density (J). Our results further illustrate the potential of the multi-colored dye sensitization concept as a powerful approach to mitigate the non-ideal optical absorption normally encountered in organic-based optoelectronic devices.
Keywords: Engineering controlled terms: Charge transfer; Dye-sensitized solar cells; Efficiency; Electromagnetic wave absorption; Light absorption; Nitrogen compounds; Optoelectronic devices; Organic solar cells; Photosensitizers; Polymer blends; Polymer solar cells; Silicon; Solar cells Characterization techniques; Dye sensitization; External quantum efficiency; Light-harvesting; Naphthalocyanines; Phthalocyanine dyes; Signal characteristic; Spectral absorptions Engineering main heading: Silicon solar cells

P- and n-type Doping in SiC Sublimation Epitaxy Using Highly Doped Substrates - Hens, Philip; Syvaejaervi, Mikael; Oehlschläger, Felix; Wellmann, Peter; Yakimova, Rositza (Materials Science Forum, 2009)
The co-doping of nitrogen and aluminum has been Studied in the sublimation epitaxy growth process. It is shown that the doping may be tuned from n-type to p-type by, effect of substrate, doping, growth face and volume of the growth crucible. The co-doped layers show a nearly ideal I-V characteristic and luminescence at room temperatur
Keywords: Compensation; Doping; Growth; Sublimation

Parasitic emission in inkjet-printed InP-based quantum dot light-emitting diodes - Hübner, Tobias; Richter, Alexander F.; Feldmann, Jochen; Brabec, Christoph; von Malm, Norwin (Organic Electronics, 2021)
Indium phosphide-based colloidal quantum dot (QD) light-emitting diodes represent a promising technology for various lighting applications. To promote this innovative technology closer to an industrialized production environment, the fabrication methods should be adapted. Hence it is necessary to replace the common spin-coating process under an inert atmosphere, by a more cost-efficient inkjet-printing process at ambient conditions. However, in our case, this transfer results in devices with limited performance and parasitic emission channels besides the desired QD emission. In this paper, we identify the physical origin of these parasitic emission channels for three different device layouts depending on the QD material as well as the number of inkjet-printed layers. For the first type of devices, a recombination process on the dopant of the electron transporting layer (ETL) as well as an exciplex formation at the interface between QDs and ETL was identified. For the next device layout, the introduction of a hole-conducting matrix embedding the QDs leads to a shift of the parasitic emission with contributions from the matrix material. Finally, the integration of a hole injection layer leads to a reduction of the undesired emission processes. For all three kinds of devices, the spacial separation of the dopant in the ETL from the QDs is a critical factor, since it directly influences the parasitic emission channels.
CRIS-Team Scopus Importer:2021-04-23
Keywords: Charge carrier balance; Colloidal quantum dot LED; Indium phosphide; Inkjet-printing; Parasitic emission

Passive cooling of large-area organic light-emitting diodes - Schwamb, Philipp; Reusch, Thilo C. G.; Brabec, Christoph (Organic Electronics, 2013)
The authors investigated passive cooling of large-area organic light-emitting diodes (OLEDs) with special focus on convective cooling. Electro-optical and thermal behaviour of large-area OLED devices are therefore modelled using finite element method (FEM) and computational fluid dynamics (CFD) simulations. Resulting temperature and luminance distributions are compared with measurement data at different driving conditions and test setups. The investigation yields that including laterally resolved convection coefficients from CFD simulations greatly improves model accuracy compared to simpler convection estimations. These findings are important for OLED and their heat spreader design especially for features like flexible, transparent, high-power and or large-area due to their specific limitations for heat spreading and or their high heat spreading requirements. © 2013 Elsevier B.V. All rights reserved.
Keywords: Computational fluid dynamics; Heat distribution; Natural convection; Organic light-emitting diode; Passive cooling; Thermal modeling

Pathways toward 30% Efficient Single-Junction Perovskite Solar Cells and the Role of Mobile Ions - Diekmann, Jonas; Caprioglio, Pietro; Futscher, Moritz H; Le Corre, Vincent Marc; Reichert, Sebastian; Jaiser, Frank; Arvind, Malavika; Perdigón-Toro, Lorena; Gutierrez-Partida, Emilio; Peña-Camargo, Francisco; Deibel, Carsten; Ehrler, Bruno; et al. (Solar RRL, 2021)
Perovskite semiconductors have demonstrated outstanding external luminescence quantum yields, enabling high power conversion efficiencies (PCEs). However, the precise conditions to advance to an efficiency regime above monocrystalline silicon cells are not well understood. Herein, a simulation model that describes efficient p-i-n-type perovskite solar cells well and a range of different experiments is established. Then, important device and material parameters are studied and it is found that an efficiency regime of 30% can be unlocked by optimizing the built-in voltage across the perovskite layer using either highly doped (10(19) cm(-3)) transport layers (TLs), doped interlayers or ultrathin self-assembled monolayers. Importantly, only parameters that have been reported in recent literature are considered, that is, a bulk lifetime of 10 mu s, interfacial recombination velocities of 10 cm s(-1), a perovskite bandgap (E-gap) of 1.5 eV, and an external quantum efficiency (EQE) of 95%. A maximum efficiency of 31% is predicted for a bandgap of 1.4 eV. Finally, it is demonstrated that the relatively high mobile ion density does not represent a significant barrier to reach this efficiency regime. The results of this study suggest continuous PCE improvements until perovskites may become the most efficient single-junction solar cell technology in the near future.
Keywords: perovskite solar cells; solar cell hysteresis; solar cell simulations; transport-layer doping

Patterning of OPV modules by ultra-fast laser - Kubis, Peter; Lucera, Luca; Guo, Fei; Spyropoulos, Georgios; Voigt, Monika; Brabec, Christoph (Laser Processing and Fabrication for Solar, Displays, and Optoelectronic Devices III, 2014)
A novel production process combining slot-die coating, transparent exible IMI (ITO-Metal-ITO) electrodes and ultra-fast laser ablation can be used for the realization of P3HT:PCBM based thin film exible OPV modules. The fast and precise laser ablation allows an overall efficiency over 3 % and a device geometric fill factor (GFF) over 95 %. Three functional layers can be ablated using the same wavelength only with varying the laser uence and overlap. Different OPV device architectures with multilayers utilizing various materials are challenging for ablation but can be structured by using a systematical approach.
Keywords: Exible OPV module; Laser ablation; Semitransparent OPV module; Slot-die coating; Ultra-fast laser

Patterning of organic photovoltaic modules by ultrafast laser - Kubis, Peter; Li, Ning; Stubhan, Tobias; Machui, Florian; Matt, Gebhard; Voigt, Monika; Brabec, Christoph (Progress in Photovoltaics: Research and Applications, 2015)
In this paper, we demonstrate that laser patterning of organic solar cells by ultrafast laser systems (pulse length <350 fs) is an attractive process to produce photovoltaic modules with outstanding high geometric fill factors. Moreover, in terms of precision, registration, and debris generation and in terms of keeping the damage to the underneath layers at a minimum, ultrafast laser patterning with a pulse length of few hundreds of femtoseconds turns out to yield superior results. Ablation of all three different solar cell layers (electrodes (P1 and P3) and interfaces and semiconductor (P2)) is achieved with a single wavelength simply by a precise adjustment of the laser fluence and the patterning overlap. Camera positioning allows a precise registration between the various processing steps and a reduction of the width of the overall interconnection regime to the hundreds of micrometers dimension, resulting in high geometrical fill factors of over 90% for monolithically interconnected organic solar cell modules.
Keywords: Femtosecond pulses; Laser patterning; Organic solar cells; PV module

PbS nanocrystal growth and overgrowth by thiourea precursors - Heiß, Wolfgang; Killilea, Niall Andrew; Sytnyk, Mykhailo (PbS nanocrystal growth and overgrowth by thiourea precursors, 2020)
Colloidal nanocrystals from PbS are most prominent materials for applications in optoelectronic devices operating in near to mid-infrared. Traditionally, they are synthesized by a hot injection method based on bis(trimethylsilyl) sulfide used as sulfur precursor. More recently Hendricks et al. [1] introduced a library of substituted thio-ureas as sulfur precursors of varying reactivity that can be used for size tuning. We combined these two approaches and selected a disubstituted thiourea compound as sulfur precursor and show the growth [2] and overgrowth of colloidal nanocrystals with this precursor. The advantage of this method is, that we can obtain controlled growth over infinitely large substrates, over all the nanocrystal size range towards bulk material (Figure 1). With the thio-urea precursor homo-epitaxial growth is achieved as well as heteroepitaxial growth, certainly related to the lattice matching between the substrate and the overgrowing PbS crystal. The obtained materials are of excellent quality and based on the nanocrystals grown from the thiourea precursor, photoconducting devices are demonstrated with band gap energies reaching those of the bulk material. The nanocrystals were also applied in photovoltaic devices providing record like behavior, especially for relatively long wavelengths [3]. Thus the growth of optoelectronic structures form thiourea precursors in organic solvents represents a versatile and promising rout for the low cost fabrication of infrared-optoelectronic devices.<br

PEDOT:PSS-Free Polymer Non-Fullerene Polymer Solar Cells with Efficiency up to 18.60% Employing a Binary-Solvent-Chlorinated ITO Anode - Sun, Rui; Wang, Tao; Wu, Yao; Zhang, Meng; Ma, Yunlong; Xiao, Zuo; Lu, Guanghao; Ding, Liming; Zheng, Qingdong; Brabec, Christoph; Li, Yongfang; Min, Jie (Advanced Functional Materials, 2021)
Despite the tremendous development of different high-performing photovoltaic systems in non-fullerene polymer solar cells (PSCs), improving their performance is still highly demanding. Herein, an effective and compatible strategy, i.e., binary-solvent-chlorinated indium tin oxide (ITO) anode, is presented to improve the device performance of the state-of-the-art photoactive systems. Although both ODCB (1,2-dichlorobenzene) solvent- and ODCB:H2O2 (hydrogen peroxide) co-solvent-chlorinated ITO (ITO-Cl-ODCB and ITO-Cl-ODCB:H2O2) show similar optical transmittance, electrical conductivities, and work function values, ITO-Cl-ODCB:H2O2 exhibits higher Cl surface coverage and more suitable surface free energy close to the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-buffered ITO anode (ITO/PEDOT:PSS). As a direct consequence, the performance of ITO-Cl-ODCB-based PBDB-T-2F:BTP-eC9:PC71BM PSCs is comparable as the bare ITO-based devices. In contrast, the performance of ITO-Cl-ODCB:H2O2-based devices with both small and the scaled-up areas significantly surpass the ITO/PEDOT:PSS-based devices. Furthermore, detailed experimental studies are conducted linking optical property, blend morphology, and physical dynamics to find the reasons for the performance difference. By applying the ITO-Cl-ODCB:H2O2 anode to six other photovoltaic systems, the device efficiencies are enhanced by 3.6–6.2% relative to those of the ITO/PEDOT:PSS-based control devices, which validates its great application potential of co-solvent-modified ITO anode employed into PEDOT:PSS-free PSCs.
CRIS-Team Scopus Importer:2021-09-24
Keywords: blend morphology; non-fullerene acceptors; optical transmittance; polymer solar cells; surface chlorination

Perfect materials as the base for technical innovation - Wellmann, Peter (The Innovation Platform, 2020)
Keywords: materials; innovation; European Materials Research Society; SiC

Perfluoroalkylsulfonyl ammonium for humidity- resistant printing high-performance phase-pure FAPbI3 perovskite solar cells and modules - Chen, Xining; Yang, Fu; Yuan, Linhao; Huang, Shihao; Gu, Hao; Wu, Xiaoxiao; Shen, Yunxiu; Chen, Yujin; Li, Ning; Egelhaaf, Hans-Joachim; Brabec, Christoph; Zhang, Rui; et al. (Joule, 2024)
High-quality phase-pure formamidinium lead triiodide (FAPbI3) perovskite film needs to be fabricated under strict control of the surrounding atmosphere, which becomes more rigorous when large-area FAPbI3 film is involved, leading to high-performance FAPbI3 perovskite solar cells and modules predominantly carried out in an inert gas-filled atmosphere. In this work, we propose a scalable printing strategy for the large-area high-quality phase-pure FAPbI3 film under a high-humidity atmosphere (up to 75% ± 5% relative humidity) by regulating the perovskite precursor ink with a functional perfluoroalkylsulfonyl quaternary ammonium iodide. This approach decreases the energy barriers of cubic phase formation and heterogeneous nucleation, thereby regulating the FAPbI3 crystallization. The printed photovoltaic small-area cells and large-area modules achieved remarkable power conversion efficiencies of 24.37% and 22.00%, respectively. Specifically, the unencapsulated device exhibits superior operational stability with T90 > 1,060 h, ambient stability with T90 > 2,020 h, and thermal stability with T90 > 2,350 h.
CRIS-Team Scopus Importer:2024-07-19
Keywords: blade coating; film formation; humid atmosphere; perovskite crystallization; perovskite solar cells; scalable printing; solar module; stable perovskite

Performance Analysis of Pre-Cracked PV-Modules at Realistic Loading Conditions - Buerhop, Claudia; Winkler, Thilo; Fecher, Frank W.; Hauch, Jens; Bemm, Andreas; Camus, Christian; Brabec, Christoph (33rd European Photovoltaic Solar Energy Conference and Exhibition, 2017)
Damaged modules, especially with existing cell cracks, are observed quite often in PV-installations. Little knowledge exists about the stability of such pre-cracked modules under real operating conditions. Previous investigations and existing standards focus on the degradation of new, defect-free modules. This work highlights a twofold approach for performance study of pre-cracked modules: 1) artificial stressing of 20 representative pre-cracked modules with a novel load test set-up simulating snow and wind loads, 2) simulating IV-curves of cracked cells. The experimental results are discussed with respect to field exposure data. EL-images of stressed PV-modules indicate that new cracks are only initiated once a certain threshold is exceeded. Below this threshold formerly unseen cracks open, as can be seen by small changes in the EL-intensity. Power measurements yield a strong power reduction at the loaded state. Due to the solar cell being under tension the cracks open and the separated areas lose their electrical contact. Considering a break resistance in the IVcurve, simulations lead to good agreement with the observed experimental results. It is highlighted that the electric contact after unloading for the previously unloaded module recovers visibly in the ELimage as well as in the power output. Simulating real weather (operating) conditions by moderate static loading tests reveals no measurable changes in the EL-images affecting the power output.
Keywords: Degradation; Performance; Module; Cracks; EL Imaging

Performance enhancement of the p3ht/pcbm solar cells through nir sensitization using a small-bandgap polymer - Ameri, Tayebeh; Min, Jie; Li, Ning; Machui, Florian; Baran, Derya; Forster, Michael; Schottler, Kristina J.; Dolfen, Daniel; Scherf, Ullrich; Brabec, Christoph (Advanced Energy Materials, 2012)

Performance Evaluation of Semitransparent Perovskite Solar Cells for Application in Four-Terminal Tandem Cells - Kirchartz, Thomas; Korgitzsch, Sophie; Hüpkes, Jürgen; Ramírez Quiroz, César Omar; Brabec, Christoph (ACS Energy Letters, 2018)
The efficiency of perovskite-based tandem solar cells and the respective efficiency gain over the single-junction operation of the bottom cell strongly depend on the performance of the component cells. Thus, a fair comparison of reported top cells is difficult. We therefore compute the tandem cell efficiency for the combination of several semitransparent perovskite top solar cells and crystalline silicon or chalcopyrite bottom cells from the literature. We focus on four-terminal configurations but also estimate and discuss the differences between four- and two-terminal configurations. For each top cell, we thereby determine the tandem cell performance as a function of the bottom cell efficiency, which results in a linear relationship. From these data, we extract two parameters to quantify the suitability of the top cell: (i) the slope of the tandem vs. bottom cell efficiency, which is the effective transparency of the top cell, and (ii) the tandem cell efficiency for a targeted bottom cell. These two figures of merit were calculated for a representative set of bottom cells and may serve for comparison of semitransparent perovskite top cells in the future. Copyright © 2018 American Chemical Societ
Keywords: Copper compounds; Efficiency; Perovskite; Perovskite solar cells

Performance of a Module and Defect Detection Algorithm for Aerial Infrared Images as a Function of the Flying Altitude - Dalsaß, Manuel; Deitsch, Sergiu; Luchscheider, Philipp; Gallwitz, Florian; Brabec, Christoph (32nd European Photovoltaic Solar Energy Conference and Exhibition, 2016)
n recent years, remote-controlled unmanned air vehicles (UAV) have been used increasingly to attain aerial infrared (aIR) recordings of large photovoltaic (PV) power plants with which thermally suspicious PV modules can be identified and localized. But so far, aIR recordings are still analyzed mostly manually – a process which can be extremely time-consuming and error-prone. In this context, the performance of a module and defect detection algorithm for aIR images is evaluated as a function of the flying altitude. For this purpose, aIR recordings of a PV power plant are generated from six flying altitudes from 5 m to 30 m above ground level by means of a drone measurement system and used as data input for the image processing tool. The results indicate optimal flying altitudes for both module and defect detection. However, the most suitable flying altitude may be a compromise with further conditions. The findings of this work will advance future automatic monitoring and early detection systems.<br
Keywords: Monitoring; Aerial Thermography; PV Module Defects; Automatic Detection

Perovskite Solar Cells: A brief Introduction and some Remarks - Almora Rodriguez, Osbel; Vaillant-Roca, Lídice; Garcia-Belmonte, Germà (Revista Cubana de Fisica, 2017)
During the last four years, the spectacular and unprecedented rise of so-called perovskite solar cells (PSCs) in conversion efficiency with low-cost manufacturing processes. The inclusion of perovskite type absorber materials, typically CH3NH3PbI3, has been a key factor in the development of this emerging technology. However, many poorly understood aspects of its operating modes. This paper provides a brief introduction to the structure, materials and characteristics of PSCs. In addition, the state-of-the-art of several subjects of interest is discussed, as well as the hysteresis phenomenon of current-voltage curves.<br
Keywords: Efficiency and performance of solar cells; Organic-inorganic hybrid nanostructures; Semiconductors thin films

Perovskite Solar Cells and Light Emitting Diodes: Materials Chemistry, Device Physics and Relationship - Yuan, Ligang; Xue, Qifan; Wang, Feng; Li, Ning; Waterhouse, Geoffrey I.N.; Brabec, Christoph; Gao, Feng; Yan, Keyou (Chemical Reviews, 2025)
Solution-processed perovskite solar cells (PSCs) and perovskite light emitting diodes (PeLEDs) represent promising next-generation optoelectronic technologies. This Review summarizes recent advancements in the application of metal halide perovskite materials for PSC and PeLED devices to address the efficiency, stability and scalability issues. Emphasis is placed on material chemistry strategies used to control and engineer the composition, deposition process, interface and micro-nanostructure in solution-processed perovskite films, leading to high-quality crystalline thin films for optimal device performance. Furthermore, we retrospectively compare the device physics of PSCs and PeLEDs, their working principles and their energy loss mechanisms, examining the similarities and differences between the two types of devices. The reciprocity relationship suggests that a great PSC should also be a great PeLED, motivating the search for interconverting photoelectric bifunctional devices with maximum radiative recombination and negligible non-radiative recombination. Specific requirements of PSCs and PeLEDs in terms of bandgap, thickness, band alignment and charge transport to achieve this target are discussed in detail. Further challenges and issues are also illustrated, together with prospects for future development. Understanding these fundamentals, embracing recent breakthroughs and exploring future prospects pave the way toward the rational design and development of high-performance PSC and PeLED devices.
CRIS-Team Scopus Importer:2025-05-30

Perovskite solar cells fabricated using dicarboxylic fullerene derivatives - Erten-Ela, Sule; Chen, Haiwei; Kratzer, Andreas; Hirsch, Andreas; Brabec, Christoph (New Journal of Chemistry, 2016)
Perovskite solar cells were first fabricated in dye sensitized solar cells. But also, perovskite hybrid solar cells were demonstrated to be among the most promising candidates within the emerging photovoltaic materials with their high power conversion efficiencies and low-cost fabrication. In this work, we design and synthesize a novel benzoic acid fullerene bis adduct material (BAFB) for use in perovskite hybrid organic-inorganic solar cells. The obtained maximum efficiency is reported to be 9.63% using a novel benzoic acid fullerene bis adduct (BAFB) for perovskite heterojunction solar cells.

Perspectives of solution epitaxially grown defect tolerant lead-halide-perovskites and lead-chalcogenides - Afify, Hany A.; Sytnyk, Mykhailo; Zhou, Shuyu; Osvet, Andres; Brabec, Christoph; Korczak, Jedrzej; Szczerbakow, Andrzej; Story, Tomasz; Heiß, Wolfgang (Applied Physics Letters, 2021)
Lead-chalcogenides and lead-halide-perovskites exhibit similar physical properties, which can be summarized as defect tolerant behavior. While the lead-chalcogenides have been pioneering materials in vapor phase epitaxy, metal-halide-perovskites offer the possibility for epitaxial growth from solutions by techniques such as spin or drop casting. The obtained microstructures show promising optical properties, and in a showcase example of formamidinium-lead-tribromide on lead sulfide, we show first lasing results. These results open up several perspectives for solution epitaxial structures, including electrically pumped quantum devices, demanded not only for lighting but also for quantum information technology, which would be another milestone achievement for metal-halide semiconductors.
CRIS-Team WoS Importer:2021-12-17

Phase diagram and stability of mixed-cation lead iodide perovskites: A theory and experiment combined study - Xu, Zhengwei; Zhao, Yicheng; Zhang, Jiyun; Chen, Keqiu; Brabec, Christoph; Feng, Yexin (Physical Review Materials, 2020)
Alloying structurally similar perovskites to form mixed-cation lead iodide perovskites, e.g., Cs(x)FA((1-x))PbI(3), MA(x)FA((1-x))PbI(3), and Cs(x)MA(y)FA((1-x-y)) PbI3, could improve the performance of perovskite-based solar cells and light-emitting diodes. However, a phase diagram of them and a clear understanding of the underlying atomic-scale mechanism are still lacking. Using ab initio calculations combined with high-throughput experimentation, we demonstrate the phase diagram of mixed-cation lead iodide perovskites. Only a small proportion of monovalent cations (Cs+/Rb+/MA(+)) could be incorporated into the FAPbI(3)/MAPbI(3) matrix; otherwise it will be separated into delta-CsPbI3, delta-RbPbI3, MAI, etc. The smaller the radius of doping cations, the harder it is to incorporate them into a perovskite lattice and the easier it is to stabilize the perovskite phase. In FAPbI(3)-based multication perovskites, moreover, over 10 mol % alloying is needed to convert delta phase to alpha phase at room temperature. The combined upper and lower limits for doping concentration restrict the appropriate alloying ratio to a narrow window. We further plot the relative energy diagram for triple-cation perovskite Cs(x)MA(y)FA((1-x-y))PbI(3), which reveals the ideal doping ratio for uniform stable alloying. This theory-experiment-combined study provides a clear microscopic picture of phase stability and segregation for mixed-cation perovskite solids.
CRIS-Team WoS Importer:2020-10-09

Phase-Field Simulation of Liquid-Vapor Equilibrium and Evaporation of Fluid Mixtures - Ronsin, Olivier; Jang, Dong Ju; Egelhaaf, Hans-Joachim; Brabec, Christoph; Harting, Jens (ACS Applied Materials and Interfaces, 2021)
In solution processing of thin films, the material layer is deposited from a solution composed of several solutes and solvents. The final morphology and hence the properties of the film often depend on the time needed for the evaporation of the solvents. This is typically the case for organic photoactive or electronic layers. Therefore, it is important to be able to predict the evaporation kinetics of such mixtures. We propose here a new phase-field model for the simulation of evaporating fluid mixtures and simulate their evaporation kinetics. Similar to the Hertz-Knudsen theory, the local liquid-vapor (LV) equilibrium is assumed to be reached at the film surface and evaporation is driven by diffusion away from this gas layer. In the situation where the evaporation is purely driven by the LV equilibrium, the simulations match the behavior expected theoretically from the free energy: for evaporation of pure solvents, the evaporation rate is constant and proportional to the vapor pressure. For mixtures, the evaporation rate is in general strongly time-dependent because of the changing composition of the film. Nevertheless, for highly nonideal mixtures, such as poorly compatible fluids or polymer solutions, the evaporation rate becomes almost constant in the limit of low Biot numbers. The results of the simulation have been successfully compared to experiments on a polystyrene-toluene mixture. The model allows to take into account deformations of the liquid-vapor interface and, therefore, to simulate film roughness or dewetting.
CRIS-Team Scopus Importer:2021-12-24
Keywords: evaporation; film drying; liquid-vapor equilibrium; phase-field

Phosphor Particles for Luminescent Down-Shifting in Photovoltaics: Determination of Complex Refractive Indices - Lipovsek, Benjamin; Solodovnyk, Anastasiia; Riedel, Daniel; Osvet, Andres; Gast, Jessica; Stern, Edda; Forberich, Karen; Batentschuk, Miroslaw; Brabec, Christoph; Krč, Janez; Topič, Marko (33rd European Photovoltaic Solar Energy Conference and Exhibition, 2017)
We demonstrate a novel two-step method for accurate determination of the complex refractive indices of phosphor materials in particulate form. The real part of the refractive index is first measured at a number of selected wavelengths using the Becke Line method; the results are interpolated over a broader wavelength range using Sellmeier's equations. The imaginary part of the refractive index is extracted from measured optical characteristics of polymer layers embedded with phosphor particles; a specialized numerical simulation tool is employed for this purpose. Finally, using the determined complex refractive indices, we numerically investigate the potential of luminescent down-shifting applied to thin-film solution-processed solar cells. We study the influence of the main properties of luminescent down-shifting layers filled with phosphor particles, and indicate the gains in the solar cell performance that can be obtained using this approach.
Keywords: Organic Solar Cell; Photoluminescence; Photovoltaic (PV); Modelling / Modeling; Optical Properties

Photocatalysis: Hydrogen-Bonded Organic Semiconductors as Stable Photoelectrocatalysts for Efficient Hydrogen Peroxide Photosynthesis (Adv. Funct. Mater. 29/2016) - Jakešová, Marie; Apaydin, Doǧukan Hazar; Sytnyk, Mykhailo; Oppelt, Kerstin; Heiß, Wolfgang; Sariciftci, Niyazi Serdar; Głowacki, Eric Daniel (Advanced Functional Materials, 2016)
Keywords: hydrogen peroxide; hydrogen-bonded pigments; organic semiconductors; photocatalysis

Photocurrent generation and charge transport mechanism study in solution-processed CZTS thin films - Khanzada, Laraib S.; Makhdoom, Muhammad Atif; Lin, Xianzhong; Azimi, Hamed; Brabec, Christoph (Optik, 2023)
Clean, benign and economical energy harvesting is envisioned to mitigate the global climate change – originated from the burning of fossil fuels – addressing both the energy crises and environmental issues. In this work, charge transport mechanism and the dynamics of photocurrent generation in thin films of CZTS nanocrystals is studied. A liquid phase processing route – owing to low temperature – has been adopted using a molecular based precursor solution for the in-situ development of CZTS crystals during film deposition. Results shows that charge carriers follow band conduction and nearest-neighbor hopping (NNH) mechanisms at elevated temperatures while Mott variable-range hopping (VRH) is the dominating phenomenon at low temperatures which are analogues to expensive vacuum-deposited kieserite thin films behavior.
CRIS-Team Scopus Importer:2023-01-06
Keywords: Conductivity; CZTS; Kesterite; Persistent photoconductivity; Solar Cell; Thin Film

Photodegradation of Organic Solar Cells under Visible Light and the Crucial Influence of Its Spectral Composition - Weitz, Paul; Wortmann, Jonas; Liu, Chao; Wen, Tian Jiao; Li, Chang Zhi; Heumüller, Thomas; Brabec, Christoph (ACS Applied Materials and Interfaces, 2024)
While wavelength-dependent photodegradation of organic solar cells (OSCs) under visible light is typically discussed in terms of UV/blue light-activated phenomena, we recently demonstrated wavelength-dependent degradation rates up to 660 nm for PM6:Y6. In this study, we systematically investigated this phenomenon for a broad variety of devices based on different donor:acceptor combinations. We found that the spectral composition of the light used for degradation, tuned in a spectral range from 457 to 740 nm and under high irradiances of up to 30 suns, has a crucial influence on the device stability of almost all tested semiconductors. The relevance of this phenomenon was investigated in the context of simulated AM1.5 illumination with metal halide lamps and white LEDs. It is concluded that the current stability testing protocols in OSC research have to be adjusted to account for this effect to reveal the underlying physics of this still poorly understood mechanism.
CRIS-Team Scopus Importer:2024-07-26
Keywords: acceleration; high irradiances; organic solar cells; stability; wavelength dependence

Photodegradation of Si-PCPDTBT:PCBM active layer for organic solar cells applications: A surface and bulk investigation - Tournebize, Aurelien; Seck, Mamadou; Vincze, Andrej; Distler, Andreas; Egelhaaf, Hans-Joachim; Brabec, Christoph; Rivaton, Agnes; Peisert, Heiko; Chasse, Thomas (Solar Energy Materials and Solar Cells, 2016)
The photooxidation of a polymer blend film used in efficient solar cells based on poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30- d]silole)-2,6-diylalt-(2,1,3-benzothiadiazole)-4,7-diyl], (Si-PCPDTBT) and [6,6]-phenyl-C71-butyric acid methyl (PC70BM) has been investigated. A set of experiments from complementary techniques was developed to monitor the modifications during ageing that occur not only in the bulk but also at the surface. The surface analyses were performed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), and the bulk analyses by UV–visible spectroscopy and infrared (IR) spectroscopy. The results reveal that the silicon bridge atom is the first target of degradation. We have identified the existence of a photooxidation profile within the 300 nm thick film. Such a heterogeneous degradation was confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling.
Keywords: Active layer; Degradation; Organic solar cell; Photooxidation profile

Photoinduced degradation of methylammonium lead triiodide perovskite semiconductors - Tang, Xiaofeng; Brandl, Marco; May, Benjamin; Levchuk, Ievgen; Hou, Yi; Richter, Moses; Chen, Haiwei; Chen, Shi; Kahmann, Simon; Osvet, Andres; Maier, Florian; Steinrück, Hans-Peter; et al. (Journal of Materials Chemistry A, 2016)
Photoinduced degradation is a critical obstacle for the real application of novel semiconductors for photovoltaic applications. In this paper, the photoinduced degradation of CHNHPbI in a vacuum and air (relative humidity 40%) is analyzed by ex situ and advanced in situ technologies. Without light illumination, CHNHPbI films slowly degrade under vacuum and air within 24 hours. However, we find that CHNHPbI converts to metallic lead (Pb) when exposed to vacuum and light illumination. Further, a series of lead salts (e.g. PbO, Pb(OH) and PbCO) are formed when CHNHPbI is degraded under environmental conditions, i.e. under the combination of light, oxygen and moisture. Photoinduced degradation is found to be determined by the environmental atmosphere as CHNHPbI films remain very stable under nitrogen conditions. The results from vacuum conditions underpin that the high volatility of the organic component (CHNHI) is in conflict with reaching excellent intrinsic stability due to its role in creating ion vacancies. The degradation in air suggests that both oxygen and water contribute to the fast photodecomposition of CHNHPbI into lead salts rather than water alone. Given these basic yet fundamental understandings, the design of hydrophobic capping layers becomes one prerequisite towards long-term stable perovskite-based devices.
Keywords: Engineering controlled terms: Lead; Perovskite; Salts; Slow light Environmental conditions; Intrinsic stability; Light illumination; Organic components; Photo-decomposition; Photoinduced degradation; Photovoltaic applications; Real applications Engineering main heading: Photodegradation

Photoluminescence for Defect Detection on Full-Sized Photovoltaic Modules - Doll, Bernd; Hepp, Johannes; Hoffmann, Mathis; Schuler, Rene; Buerhop-Lutz, Claudia; Peters, Ian Marius; Hauch, Jens; Maier, Andreas; Brabec, Christoph (IEEE Journal of Photovoltaics, 2021)
Cost-effective, fast, and nondestructive on-site characterization of photovoltaic plants is required to determine countermeasures against power loss, defects, or safety problems. Methods with small impact on the operation and a high throughput, such as infrared thermography (IR), or methods with high resolution for detailed defect information, such as electroluminescence (EL) imaging, are expedient. To combine high resolution and high throughput, we propose to use photoluminescence (PL) as an outdoor characterization method for full-sized module imaging. With PL imaging as with IR imaging, no electrical contact is necessary, yet image resolution is on par with EL images. Our outdoor PL setup features an excitation source with 18 broadband, white, high power, chip on board LEDs coupled with low-cost short pass filters. This setup is suitable for indium gallium arsenide and silicon detectors. Here, we compare the visibility of common defects, including short-circuited bypass diodes, cracks, potential induced degradation, snail trails, ethylene-vinyl acetate degradation, and interconnection failures, in PL images to that in state-of-the-art imaging techniques IR, EL, and ultraviolet fluorescence. We find that out of these seven defects, five can be detected well, cell cracks under certain conditions, and interconnection failures not at all. We discuss how different techniques are complementary to enable better defect detection.
CRIS-Team Scopus Importer:2021-09-17
Keywords: Cameras; Defect detection; Detectors; fluorescence; Image resolution; Imaging; luminescence; photoluminescence; photovoltaic (PV) systems; Silicon; silicon (Si); Throughput; Tools

Photoluminescence properties of thermographic phosphors YAG:Dy and YAG:Dy, Er doped with boron and nitrogen - Chepyga, Liudmyla M.; Jovicic, Gordana; Vetter, Andreas; Osvet, Andres; Osvet, Andres; Brabec, Christoph; Batentschuk, Miroslaw (Applied Physics B-Lasers and Optics, 2016)
This paper investigates Dy-doped and Dy, Er-co-doped yttrium aluminum garnets (YAG) with the admixture of boron nitride with the aim of using them as efficient thermographic phosphors at high temperatures. The phosphors were synthesized using a conventional high-temperature solid-state method. The influence of two fluxes, BO and LiF/NHF, and the effect of activator and coactivator concentrations were investigated. Additionally, the effect of B and N substituting for Al and O ions, respectively, in the YAG:Dy co-doped with Er was studied for the first time. The changes in the host lattice led to a much stronger photoluminescence compared with the samples without B and N substitution. The admixture of BN also improves the thermal sensitivity of the YAG:Dy and YAG:Dy, Er thermographic phosphor
Keywords: Engineering controlled terms: Erbium; Light emission; Photoluminescence Coactivators; High temperature; High temperature solid state methods; Host lattice; Photoluminescence properties; Thermal sensitivity; Thermographic phosphors; Yttrium aluminum garnet Engineering main heading: Phosphors

Photoluminescence study of in-situ rare earth doped PVT-grown SiC single crystals - Schmitt, Holger; Müller, Ralf; Maier, M.; Winnacker, Albrecht; Wellmann, Peter (Materials Science Forum, 2005)
Several SiC bulk crystals were grown with erbium and ytterbium as doping materials. Erbium contents determined by secondary ion mass spectroscopy (SIMS) ranged from 1.2 (.) 10(14) cm(-3) to 1.04 (.) 10(15) cm(-3), while ytterbium contents were below SIMS detection limit. Photoluminescence (PL) investigations of the characteristic 4f-4f-transition lines revealed a reduced luminescence yield in highly nitrogen and aluminum co-doped samples. Also, samples without intentional co-doping grown on the C-face showed less luminescence intensity than those grown on the Si-face. A stabilizing effect of erbium doping on the 4H polytype was observe
Keywords: Erbium; Photoluminescence (PL); PVT Method; Rare Earth Doping; SiC Bulk Growth

Photoluminescence topography of fluorescent SiC and its corresponding source crystals - Wilhelm, Martin; Kaiser, Michl; Jokubavicius, Valdas; Syväjärvi, Mikael; Ou, Yiyu; Ou, Haiyan; Wellmann, Peter (Materials Science Forum, 2013)
The preparation and application of co-doped polycrystalline SiC as source in sublimation growth of fluorescent layers is a complex topic. Photoluminescence topographies of luminescent 6H-SiC layers and their corresponding source crystals have been studied in order to investigate the dependence of the epitaxial growth on the source material. It is shown that the homogeneity concerning the dopant incorporation and the layer luminescence intensity does not depend on the characteristics of the PVT grown source material. Therefore co-doped polycrystalline SiC is a promising source material in fast sublimation growth of luminescent 6H-Si
Keywords: luminescent SiC; 6H-SiC; epitaxy; photoluminescence topography

Photoluminescence-topography of the p-type Doped SiC Wafers for Determination of Doping Inhomogeneity - Oehlschläger, Felix; Juillaguet, Sandrine; Peyre, Herve; Calmassel, Jean; Wellmann, Peter (Materials Science Forum, 2009)
Photoluminescence(PL)-topography is a powerful method to determine the charge carrier concentration of SiC-wafers. The following, work describes the development of a PL-topography method for the determination of charge carrier distribution in p-type SiC and shows the correlation of PL-Intensity and charge carrier concentration. With this setup it is possible to characterize wafers up to a size of 2" at room- and low temperature in a non-destructive wa
Keywords: Absorption Measurements; Determination of Doping Inhomogeneity; Mapping; Photoluminescence-Topography; PL

Photoluminescent and storage properties of photostimulable core/shell type silicate nanoparticles - Osvet, Andres; Batentschuk, Miroslaw; Milde, Moritz; Lundt, Nils; Gellermann, Carsten; Dembski, Sofia; Winnacker, Albrecht; Brabec, Christoph (Physica Status Solidi (C) Current Topics in Solid State Physics, 2013)
We have prepared hybrid luminescent nanoparticles with amorphous SiO cores and crystalline ZnSiO:Mnshells by sol-gel technology. Under UV excitation the shells of the particles emit green Mn-related luminescence at 525 nm. In addition, under the UV excitation at least two kinds of traps are filled with electrons, contributing to a long afterglow and photostimulated luminescence. The paper presents the details of the charging and stimulation characteristics of the photostimulated luminescence. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Luminescent core/shell nanoparticles; Nanophosphors; Photostimulated luminescence; Zn2SiO4:Mn2+

Photophysical and electronic properties of bismuth-perovskite shelled lead sulfide quantum dots - Abdu-Aguye, Mustapha; Bederak, Dmytro; Kahmann, Simon; Killilea, Niall Andrew; Sytnyk, Mykhailo; Heiß, Wolfgang; Loi, Maria Antonietta (Journal of Chemical Physics, 2019)
Metal halide perovskite shelled quantum dot solids have recently emerged as an interesting class of solution-processable materials that possess the desirable electronic properties of both quantum dots and perovskites. Recent reports have shown that lead sulfide quantum dots (PbS QDs) with perovskite ligand-shells can be successfully utilized in (opto)electronic devices such as solar cells, photoconductors, and field-effect transistors (FETs), a development attributed to the compatibility of lattice parameters between PbS and certain metal halide perovskites that results in the growth of the perovskite shell on the PbS QDs. Of several possible perovskite combinations used with PbS QDs, bismuth-based variants have been shown to have the lowest lattice mismatch and to display excellent performance in photoconductors. However, they also display photoluminescence (PL), which is highly sensitive to surface defects. In this work, we present an investigation of the transport and optical properties of two types of bismuth-based perovskite (MA3BiI6 and MA3Bi2I9) shelled PbS QDs. Our photophysical study using temperature-dependent PL spectroscopy between 5 and 290 K indicates that the PL efficiency of the reference oleic acid (OA) capped samples is much higher than that of the Bi-shelled ones, which suffer from traps, most likely formed at their surfaces during the phase-transfer ligand exchange process. Nevertheless, the results from electrical measurements on FETs show the successful removal of the native-OA ligands, displaying electron dominated transport with modest mobilities of around 10-3 cm2 [V s]-1 - comparable to the reported values for epitaxial Pb-based shelled samples. These findings advance our understanding of perovskite shelled QD-solids and point to the utility of these Bi-based variants as contenders for photovoltaic and other optoelectronic applications.
CRIS-Team Scopus Importer:2019-12-17

Photophysics of Molecular-Weight-Induced Losses in Indacenodithienothiophene-Based Solar Cells - Gasparini, Nicola; Katsouras, Athanasios; Prodromidis, Mamantos I.; Avgeropoulos, Apostolos; Baran, Derya; Salvador, Michael Filipe; Fladischer, Stefanie; Spiecker, Erdmann; Chochos, Christos L.; Ameri, Tayebeh; Brabec, Christoph (Advanced Functional Materials, 2015)
The photovoltaic performance and optoelectronic properties of a donor-acceptor copolymer are reported based on indacenodithienothiophene (IDTT) and 2,3-bis(3-(octyloxy)phenyl)quinoxaline moieties (PIDTTQ) as a function of the number-average molecular weight (Mn). Current-voltage measurements and photoinduced charge carrier extraction by linear increasing voltage (photo-CELIV) reveal improved charge generation and charge transport properties in these high band gap systems with increasing Mn, while polymers with low molecular weight suffer from diminished charge carrier extraction because of low mobility-lifetime (μτ) product. By combining Fourier-transform photocurrent spectroscopy (FTPS) with electroluminscence spectroscopy, it is demonstrate that increasing Mn reduces the nonradiative recombination losses. Solar cells based on PIDTTQ with Mn = 58 kD feature a power conversion efficiency of 6.0% and a charge carrier mobility of 2.1 × 10-4 cm2 V-1 s-1 when doctor bladed in air, without the need for thermal treatment. This study exhibits the strong correlations between polymer fractionation and its optoelectronics characteristics, which informs the polymer design rules toward highly efficient organic solar cells.
Keywords: charge transport; energetic losses; indacenodithienothiophene; molecular weight; organic solar cells

Photostability of Fullerene and Non-Fullerene Polymer Solar Cells: The Role of the Acceptor - Doumon, Nutifafa Y.; Dryzhov, Mikhail V; Houard, Félix V; Le Corre, Vincent Marc; Rahimi Chatri, Azadeh; Christodoulis, Panagiotis; Koster, L Jan Anton (ACS Applied Materials and Interfaces, 2019)
Recently, the advent of non-fullerene acceptors (NFAs) made it possible for organic solar cells (OSCs) to break the 10% efficiency barrier hardly attained by fullerene acceptors (FAs). In the past five years alone, more than hundreds of NFAs with applications in organic photovoltaics (OPVs) have been synthesized, enabling a notable current record efficiency of above 15%. Hence, there is a shift in interest toward the use of NFAs in OPVs. However, there has been little work on the stability of these new materials in devices. More importantly, there is very little comparative work on the photostability of FA versus NFA solar cells to ascertain the pros and cons of the two systems. Here, we show the photostability of solar cells based on two workhorse acceptors, in both conventional and inverted structures, namely, ITIC (as NFA) and [70]PCBM (as FA), blended with either PBDB-T or PTB7-Th polymer. We found that, irrespective of the polymer, the cell structure, or the initial efficiency, the [70]PCBM devices are more photostable than the ITIC ones. This observation, however, opposes the assumption that NFA solar cells are more photochemically stable. These findings suggest that complementary absorption should not take precedence in the design rules for the synthesis of new molecules and there is still work left to be done to achieve stable and efficient OSCs.
Keywords: Fullerene derivatives; Non-Fullerene acceptors; Degradation; Organic solar cells; Photostability

Photostimulable fluorescent nanoparticles for biological imaging - Osvet, Andres; Milde, Moritz; Dembski, Sofia; Rupp, Sabine; Gellermann, Carsten; Batentschuk, Miroslaw; Brabec, Christoph; Winnacker, Albrecht (2011 MRS Spring Meeting, 2012)
Spherical monodisperse core/shell-type nanoparticles, comprising an amorphous SiO core coated with a luminescent phosphor layer were synthesized by the modified Pechini processes. The sol-gel method allows covering the 50-500 nm core particles with different inorganic phosphor layers of about 10 nm thickness, doped with rare-earth or transition metal ions which determine the luminescent properties. Particles comprising a Zn SiO shell, doped with Mn ions, are not only fluorescent under UV irradiation (260 nm), but store the activation energy by trapping electrons/holes at lattice defects. This energy is released as phosphorescence in the time scale of seconds and minutes, or as photostimulated luminescence under the excitation of red light (650 nm). Traps related to these processes are different, and their concentration is affected by the preparation conditions of the particles. © 2011 Materials Research Society.
Keywords: Biological imaging; Core particles; Fluorescent nanoparticles; Luminescent property; Monodisperse; Pechini process; Phosphor layers; Photo stimulated luminescence; Preparation conditions; Red light; Time-scales; Trapping electrons; UV irradiation Engineering controlled terms: Activation energy; Biological materials; Coated materials; Fluorescence; Light emission; Manganese; Manganese compounds; Metal ions; Nanoparticles; Phosphors; Silicon compounds; Sol-gel process; Transition metals Engineering main heading: Amorphous materials

Photovoltaic limitations of BODIPY:fullerene based bulk heterojunction solar cells - Baran, Derya; Tuladhar, Sachetan; Economopoulos, Solon P.; Neophytou, Marios; Savva, Achilleas; Itskos, Grigorios; Othonos, Andreas; Bradley, Donal D C; Brabec, Christoph; Nelson, Jenny; Choulis, Stelios A. (Synthetic Metals, 2017)
The photovoltaic performance of blends of a series of 4,4′-Difluoro-4-bora-3a,4a-diaza-s-indacenes)-based (BODIPY) conjugated polymers donors with fullerene electron acceptors is investigated. Despite the high Voc values observed, solar cell device yields relatively low power conversion efficiencies. Our study takes into account the materials’ structure-property relationship, light harvesting capabilities, charge transport, collection properties and morphological characteristics to elucidate factors affecting the photovoltaic performance in this class of polymers. We show that elimination of low molecular weight species and suitable electrodes for hole collection can be used to overcome some of the observed limitations on photovoltaic performance.
Keywords: BODIPY polymer donors; Charge transport; Collection; Fractionation process; Morphology; Organic solar cells

Photovoltaic loss analysis of inkjet-printed polymer solar cells using pristine solvent formulations - Hoth, C. N.; Schilinsky, P.; Choulis, S. A.; Brabec, Christoph (Macromolecular Symposia, 2010)
We have recently reported inkjet-printed organic solar cells with a record power conversion efficiency of 3.5%. In this contribution, we present our first trials to process a functional active layer of a polymer:fullerene bulk-hetero junction solar cell by inkjet printing using tetralene as a solvent. Solar cells with the inkjet-printed active layer based on pristine tetralene show calibrated AM1.5 power conversion efficiency (PCE) of around 1.3% over comparable doctor bladed cells with PCE of 3.3%. Analysis in terms of one-diode equivalent circuit combined with current-voltage characteristics of the devices as a function of light intensity and voltage were performed to reveal the dominant loss mechanisms of the inkjet printed solar cells using tetralene solvent formulation. The loss analysis described in this paper helps us to identify the device performance limitations and to design methods to improve the performance of inkjet printed organic solar cells. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Device loss analysis; Fullerene; Inkjet printing; Morphology; Organic photovoltaics; Polymer; Polymer solar cells; Solvent formulations

Photovoltaic properties of benzotriazole containing alternating donor-acceptor copolymers: Effect of alkyl chain length - Baran, Derya; Balan, Abidin; Stubhan, Tobias; Ameri, Tayebeh; Toppare, Levent; Brabec, Christoph (Synthetic Metals, 2012)
A series of variable alkyl chain length substituted donor-acceptor (D-A) conjugated polymers with thiophene ring as the donor and benzotriazole moiety as the acceptor has been investigated in bulk heterojunction solar cells. The optical and electrochemical properties showed that the absorption onsets and the energy levels of the copolymers were not affected by alkyl substitution revealing 1.9 eV of optical band gap. The morphologies of the blend film can be fine-tuned by increasing the chain length attached to the benzotriazole unit. Photovoltaic devices were fabricated using (6,6)-phenyl-C-butyric acid methyl ester (PCBM) and (6,6)-phenyl-C-butyric acid methyl ester (PCBM) as the acceptors. The maximum performance was achieved with a V of 0.75 V, J of 3.70 mA/cm , FF of 45% and a PCE of 1.23% for the PTBT-DA10:PCBM device using 1:4 w/w ratio in chlorobenzene (CB). © 2012 Elsevier B.V. All rights reserved.
Keywords: Benzotriazole; Donor-acceptor polymers; Effect of chain length; Organic photovoltaics; Solar cell

Photovoltaic properties of thin film heterojunctions with cupric oxide absorber - Dimopoulos, T.; Peic, A.; Muellner, P.; Neuschitzer, Markus; Resel, Roland; Abermann, S.; Postl, M.; List, E. J. W.; Yakunin, S.; Heiß, Wolfgang; Brueckl, H. (Journal of Renewable and Sustainable Energy, 2013)
In this work, we report on the fabrication, characterization, and photovoltaic properties of sputter-deposited, thin film heterojunctions combining p-type cupric oxide (CuO) absorber with n-type ZnO. The structural investigation reveals highly crystalline, columnar growth of the layers and confirms that the absorber's phase is purely CuO, with only negligible traces of Cu2O. The optical characterization yields for CuO an indirect bandgap of 1.2 eV and a direct optical transition at approximately 3 eV. The short circuit current, open circuit voltage, fill factor, and power conversion efficiency of the heterojunction solar cells were extracted as a function of the CuO thickness under AM1.5G (1 kW/m(2)) illumination. From the observed dependencies, we conclude that the photovoltaic performance is compromised by a restricted carrier collection efficiency, caused by the small carrier lifetime in CuO. Indeed, the carrier population is found to decay with time constants of 40 and 460 ps. A maximum power conversion efficiency of 0.08% was obtained for the solar cell with CuO thickness of 500 nm. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4791779]
Keywords: Carrier collection efficiencies; Carrier population; Heterojunction solar cells; Optical characterization; Photovoltaic performance; Photovoltaic property; Power conversion efficiencies; Structural investigation

Physical vapor growth of double position boundary free, quasi-bulk 3C-SiC on high quality 3C-SiC on Si CVD templates - Schuh, Philipp; Vecera, Philipp; Hirsch, Andreas; Syväjärvi, Mikael; Litrico, G.; La Via, F.; Mauceri, M.; Wellmann, Peter (Materials Science Forum, 2016)
We have developed and investigated the quasi-bulk crystal growth of 3C-SiC on transferred, high quality, CVD-grown templates using a sublimation sandwich technique. The 3CSiC seeding template layers grown on silicon were removed from the silicon carrier using a solution of hydrofluoric acid, nitric acid and water and glued to a SiC substrate using carbon glue. The transferred layers were used as seeding material to grow high quality, single crystalline, double position boundary free 3C-SiC crystals with thicknesses of up to 860 μm and a size of 18 x 20 mm. Analysis of the bulk-like 3C-SiC layers was carried out using Laue measurement in back-reflecting geometry, Raman spectroscopy and optical microscop
Keywords: 3C-SiC; DPBs free; Quasi-bulk; Seed transfer/stabilization; Seeded sublimation epitaxy

Physics-guided characterization and optimization of solar cells using surrogate machine learning model - Ren, Zekun; Oviedo, Felipe; Xue, Hansong; Thway, Muang; Zhang, Kaicheng; Li, Ning; Perea, Jose Dario; Layurova, Mariya; Wang, Yue; Tian, Siyu I. P.; Heumüller, Thomas; Birgersson, Erik; et al. (2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019)
Characterization, material parameter extraction and subsequent optimization of solar cell devices is a highly time‑consuming and complex procedure. In this work, we propose a method for quick extraction of limiting material parameters in solar cell devices using a surrogate, physics-embedded, neural network model. This surrogate model, implemented by an autoencoder architecture trained with a physical numerical model, allows to quickly extract the device parameters of interest at a certain process condition by using only a small number of illumination dependent current-voltage (JV) measurements. Our surrogate model adequately links material parameters at a certain process condition to final device efficiency. The model provides physical insights about the location of the best performing and robust processing conditions in solar cell devices. We test our approach with GaAs and CH3NH3PbI3 (MAPbI) perovskite solar cells. The model allows to find a set of processing conditions that maximize the performance of both GaAs and MAPbI solar cells, and analogous processing conditions that minimize solar cell variabili

Pinhole-Free Inkjet Printing Strategies for Organic Electronics - Heinrichsdobler, Armin; Roigk, Julia Christina; Schirmeier, Frank; Brabec, Christoph; Wehlus, Thomas (Advanced Materials Technologies, 2017)
Inkjet printing is a widely used technique in the field of printed electronics. Yet its reliability is limited because absent droplets induced by defective, e.g., clogged, nozzles can lead to pinholes in the printed layers causing a reduction of the quality of printed films or a breakdown of the functionality in microelectronic devices. Therefore, pinholes in inkjet-printed layers need to be avoided. In this study the origins for pinholes in inkjet-printed films are examined. It is found that single missing droplets cannot lead to pinholes but certain formations can. This paper presents the corresponding responsible combinations of defective nozzles necessary to create a pinhole. To enable a statistical approach the pinhole occurrence probabilities are computed depending on the number of broken nozzles as well as quality factors and step sizes with a Monte Carlo simulation. The model shows that by choosing the right print strategy the pinhole probability can be reduced by three orders of magnitude. Finally, a novel print strategy is suggested, which is not yet supported by default printer settings but can reduce the pinhole probability even further by a factor of over 2000 in total. This represents the smallest pinhole occurrence probability ever achieved.
Keywords: inkjet printing; Monte Carlo simulation; pinholes; printed electronics

PMMA as an effective protection layer against the oxidation of P3HT and MDMO-PPV by ozone - Früh, Andreas; Egelhaaf, Hans Joachim; Hintz, Holger; Quinones, Dustin; Brabec, Christoph; Peisert, Heiko; Chassé, Thomas (Journal of Materials Research, 2018)
The protective effect of poly(methylmethacrylate) (PMMA) cover layers against the degradation of π-conjugated polymers by ozone and photo-oxidation, respectively, has been investigated by UV/Vis spectroscopy. The PMMA films were cast from solution at thicknesses between 20 and 100 nm on top of films of poly(3-hexylthiophene) and poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]. PMMA layers of more than 65 nm in thickness reduce the oxidation rate of the π-conjugated polymers under 15 ppm of ozone in the dark by more than three orders of magnitude, whereas photo-oxidation rates under dry and humid air remain unaffected. The PMMA cover layers are hardly affected by ambient ozone over thousands of hours. Calculations of ozone and oxygen fluxes through the PMMA films reveal that ozonation rates are limited by the diffusion of ozone, whereas photo-oxidation rates are not limited by the diffusion of oxygen, due to the much larger pressure gradient of the latte
Keywords: optoelectronic; organic; oxidation

Polarons in Narrow Band Gap Polymers Probed over the Entire Infrared Range: A Joint Experimental and Theoretical Investigation - Kahmann, Simon; Fazzi, Daniele; Matt, Gebhard; Thiel, Walter; Loi, Maria A.; Brabec, Christoph (Journal of Physical Chemistry Letters, 2016)
We investigate the photoinduced absorption (PIA) spectra of the prototypical donor-acceptor polymer [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and its silicon bridged variant Si-PCPDTBT over a spectral range from 0.07 to 1.5 eV. Comparison between time-dependent density functional theory simulations of the electronic and vibrational transitions of singlet excitons, triplet excitons, polarons, and bipolarons with the experimental results proves that the observed features are due to positive polarons delocalized on the polymer chains. We find that the more crystalline Si-bridged variant gives rise to a red-shift in the transition energies, especially in the mid-infrared (MIR) spectral range and furthermore observe that the pristine polymers' responses depend on the excitation energy. Blending with PCBM, on the other hand, leads to excitation-independent PIA spectra. By computing the response properties of molecular aggregates, we show that polarons are delocalized in not only the intra- but also the interchain direction, leading to intermolecular transitions which correspond well to experimental absorption features at the lowest energies.
Keywords: Engineering controlled terms: Blending; Computation theory; Density functional theory; Energy gap; Excitons; Polarons 2; 1; 3-Benzothiadiazole; Donor-acceptor polymers; Interchain direction; Narrow band gap polymers; Photoinduced absorption; Theoretical investigations; Time dependent density functional theory; Vibrational transitions Engineering main heading: Polymers

Polycrystalline SiC as source material for the growth of fluorescent SiC layers - Kaiser, Michl; Hupfer, Thomas; Jokubavicius, Valdas; Schimmel, Saskia; Syväjärvi, Mikael; Ou, Yiyu; Ou, Haiyan; Linnarsson, Margareta K.; Wellmann, Peter (Materials Science Forum, 2013)
Polycrystalline doped SiC act as source for fluorescent SiC. We have studied the growth of individual grains with different polytypes in the source material. We show an evolution and orientation of grains of different polytypes in polycrystalline SiC ingots grown by the Physical Vapor Transport method. The grain influence on the growth rate of fluorescent SiC layers grown by a sublimation epitaxial process is discussed in respect of surface kinetic
Keywords: Silicon Carbide; Orientation; PVT; Polycrystalline; Doping

Polymer-acid-metal quasi-ohmic contact for stable perovskite solar cells beyond a 20,000-hour extrapolated lifetime - Luo, Junsheng; Liu, Bowen; Yin, Haomiao; Zhou, Xin; Wu, Mingjian; Shi, Hongyang; Zhang, Jiyun; Elia, Jack; Zhang, Kaicheng; Wu, Jianchang; Xie, Zhiqiang; Liu, Chao; et al. (Nature Communications, 2024)
The development of a robust quasi-ohmic contact with minimal resistance, good stability and cost-effectiveness is crucial for perovskite solar cells. We introduce a generic approach featuring a Lewis-acid layer sandwiched between dopant-free semicrystalline polymer and metal electrode in perovskite solar cells, resulting in an ideal quasi-ohmic contact even at elevated temperature up to 85 °C. The solubility of Lewis acid in alcohol facilitates nondestructive solution processing on top of polymer, which boosts hole injection from polymer into metal by two orders of magnitude. By integrating the polymer-acid-metal structure into solar cells, devices exhibit remarkable resilience, retaining 96% ± 3%, 96% ± 2% and 75% ± 7% of their initial efficiencies after continuous operation in nitrogen at 35 °C for 2212 h, 55 °C for 1650 h and 85 °C for 937 h, respectively. Leveraging the Arrhenius relation, we project an impressive T80 lifetime of 26,126 h at 30 °C.
CRIS-Team Scopus Importer:2024-03-15

Polymer-assisted solegel process for the preparation of photostimulable core/shell structured SiO2/Zn2SiO4:Mn2+ particles - Milde, Moritz; Dembski, Sofia; Osvet, Andres; Batentschuk, Miroslaw; Winnacker, Albrecht; Sextl, Gerhard (Materials Chemistry and Physics, 2014)
In this study the SiO2/Zn2SiO4:Mn2+ core/shell particles with diameters of about 200 nm were prepared by a modified Pechini sol–gel method. The focus of the study was on the investigation of the shell formation mechanism and on the determination of key synthesis parameters. A precise adjustment of the particle structure is crucial for optical properties. The influence of the organic additives, the pH value of the coating solution and the annealing temperature on the properties of the resulting particles was evaluated. The roles of the influential factors were studied in detail using transmission electron microscopy, X-ray diffraction, Fourier transform infrared and nuclear magnetic resonance spectroscopy. The most homogeneous coating was achieved by employing polyethylene glycol (PEG) at a pH value of 1. The application of citric acid in combination with PEG caused the formation of inhomogeneous shells. The particles showed photoluminescence and photostimulable luminescence of increasing intensities with rising annealing temperatures (1100 °C). These core/shell structured particles have the potential to serve as luminescent labeling agents for biomedical applications.
Keywords: Photostimulated luminescence; Inorganic core/shell particles; Zinc silicate; Solegel process

Polymer-based scattering layers for internal light extraction from organic light emitting diodes - Riedel, Daniel; Wehlus, Thomas; Reusch, Thilo C. G.; Brabec, Christoph (Organic Electronics, 2016)
Efficient light extraction for organic light emitting diodes (OLED) using scalable processes and low-cost materials are important prerequisites for the future commercialization of OLED lighting devices. The light-extraction technology exhibited in this paper uses polymer-based high-refractive index scattering layers processed from solution. The scatter matrix formulation incorporates two types of nanoparticles for refractive index tuning and scattering, respectively. Planarization by the same material in order to reduce surface defects was critical for achieving highly increased device yield. Highly efficient and defect-free large-area (1.8 cm) white OLED devices were fabricated on top of the scattering layer in a bottom emitter configuration. Light extraction enhancement leads to an overall efficiency gain of up to 81% for luminances of 5000 cd m.
Keywords: Internal light extraction; OLED; Organic light-emitting diodes; Planarization; Scattering layers

Polymer encapsulation impact on potential-induced degradation in PV modules revealed by a multi-modal field study - Buerhop, C.; Stroyuk, O.; Mashkov, O.; Barabash, Anastasia; Hauch, J. A.; Peters, I. M. (Solar Energy Materials and Solar Cells, 2024)
Potential-induced degradation is a significant issue affecting the performance and reliability of photovoltaic systems, particularly in large-scale installations. Understanding the mechanisms underlying the formation of potential-induced degradation is crucial for developing effective mitigation strategies and ensuring the long-term sustainability of solar energy technologies. During the inspection of a 1.2 MWp photovoltaic power plant, various imaging methods, electrical measurements, spectroscopic characterization methods, and monitoring data analyses were utilized and combined. We observed that two percent of the photovoltaic modules at the string ends exhibited the characteristic checkerboard pattern in infrared or electroluminescence imaging, implicating issues on twelve percent of all strings. This uneven distribution of potential-induced degradation-affected modules suggests additional key factors are at play. Investigations of the backsheet and ethylene-vinyl acetate using near-infrared reflectance analysis revealed that one backsheet type and two different ethylene-vinyl acetate types were present in the installed solar panels. Yet, only one type of ethylene-vinyl acetate correlated with potential-induced degradation. The same type also exhibited a higher rate of oxidative degradation. Scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed a significantly increased sodium ion content in the ethylene-vinyl acetate and glass of the potential-induced degradation-affected modules. Our findings highlight the crucial role of polymer encapsulation in the development of potential-induced degradation and underscore the importance of careful polymer selection in the design of photovoltaic module technologies.
CRIS-Team Scopus Importer:2024-09-06

Polymer-fullerene bulk-heterojunction solar cells - Brabec, Christoph; Gowrisanker, Srinivas; Halls, Jonathan J. M.; Laird, Darin; Jia, Shijun; Williams, Shawn P. (Advanced Materials, 2010)
Solution-processed bulk heterojunction organic photovoltaic (OPV) devices have gained serious attention during the last few years and are established as one of the leading next generation photovoltaic technologies for low cost power production. This article reviews the OPV development highlights of the last two decades, and summarizes the key milestones that have brought the technology to today's efficiency performance of over 7%. An outlook is presented on what will be required to drive this young photovoltaic technology towards the next major milestone, a 10% power conversion efficiency, considered by many to represent the efficiency at which OPV can be adopted in wide-spread applications. With first products already entering the market, sufficient lifetime for the intended application becomes more and more critical, and the status of OPV stability as well as the current understanding of degradation mechanisms will be reviewed in the second part of this article. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Polymer: Nonfullerene Bulk Heterojunction Solar Cells with Exceptionally Low Recombination Rates - Gasparini, Nicola; Salvador, Michael Filipe; Heumüller, Thomas; Richter, Moses; Classen, Andrej; Shrestha, Shreetu; Matt, Gebhard; Holliday, Sarah; Strohm, Sebastian; Egelhaaf, Hans Joachim; Wadsworth, Andrew; Baran, Derya; et al. (Advanced Energy Materials, 2017)
Organic semiconductors are in general known to have an inherently lower charge carrier mobility compared to their inorganic counterparts. Bimolecular recombination of holes and electrons is an important loss mechanism and can often be described by the Langevin recombination model. Here, the device physics of bulk heterojunction solar cells based on a nonfullerene acceptor (IDTBR) in combination with poly(3-hexylthiophene) (P3HT) are elucidated, showing an unprecedentedly low bimolecular recombination rate. The high fill factor observed (above 65%) is attributed to non-Langevin behavior with a Langevin prefactor (β/βL) of 1.9 × 10-4. The absence of parasitic recombination and high charge carrier lifetimes in P3HT:IDTBR solar cells inform an almost ideal bimolecular recombination behavior. This exceptional recombination behavior is explored to fabricate devices with layer thicknesses up to 450 nm without significant performance losses. The determination of the photoexcited carrier mobility by time-of-flight measurements reveals a long-lived and nonthermalized carrier transport as the origin for the exceptional transport physics. The crystalline microstructure arrangement of both components is suggested to be decisive for this slow recombination dynamics. Further, the thickness-independent power conversion efficiency is of utmost technological relevance for upscaling production and reiterates the importance of understanding material design in the context of low bimolecular recombination. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Charge recombination; Charge transport; Langevin; Nonfullerene acceptor; Organic photovoltaic devices

Polyvinyl Alcohol and Nano-Clay Based Solution Processed Packaging Coatings - Chandio, Ali Dad; Channa, Iftikhar Ahmed; Rizwan, Muhammad; Akram, Shakeel; Javed, Muhammad Sufyan; Siyal, Sajid Hussain; Saleem, Muhammad; Makhdoom, Atif; Ashfaq, Tayyaba; Khan, Safia; Hussain, Shahid; Albaqami, Munirah D.; et al. (Coatings, 2021)
Cost-effective, clean, highly transparent, and flexible as well as a coatable packaging material is envisioned to solve or at least mitigate quality preservation issues of organic materials, originating from moisture interaction under ambient conditions. Liquid phase processing of packaging coatings using nano-clay and polyvinyl alcohol (PVOH) has been developed and reported. Detailed analysis of the developed coating revealed moisture permeability of 2.8 x 10(-2) g center dot cm/m(2)center dot day at 40 degrees C and 85% relative humidity (RH), which is in close accordance with Bharadwaj's theoretical permeability model. Moreover, the developed coatings are not only more than 90% transparent, when exposed to white light, but also exhibit excellent flexibility and even after going through 10,000 bending cycles maintained the same blocking effect against moisture.
CRIS-Team WoS Importer:2021-09-10

Power Electronic Semiconductor Materials for Automotive and Energy Saving Applications – SiC, GaN, Ga2O3, and Diamond - Wellmann, Peter (Zeitschrift für Anorganische und Allgemeine Chemie, 2017)
Power electronics belongs to the future key technologies in order to increase system efficiency as well as performance in automotive and energy saving applications. Silicon is the major material for electronic switches since decades. Advanced fabrication processes and sophisticated electronic device designs have optimized the silicon electronic device performance almost to their theoretical limit. Therefore, to increase the system performance, new materials that exhibit physical and chemical properties beyond silicon need to be explored. A number of wide bandgap semiconductors like silicon carbide, gallium nitride, gallium oxide, and diamond exhibit outstanding characteristics that may pave the way to new performance levels. The review will introduce these materials by (i) highlighting their properties, (ii) introducing the challenges in materials growth, and (iii) outlining limits that need innovation steps in materials processing to outperform current technologie
Keywords: Diamond; Energy applications; Gallium nitride; Gallium oxide; Silicon carbide

Power-Law Density of States in Organic Solar Cells Revealed by the Open-Circuit Voltage Dependence of the Ideality Factor - Saladina, Maria; Wöpke, Christopher; Göhler, Clemens; Ramirez, Ivan; Gerdes, Olga; Liu, Chao; Li, Ning; Heumüller, Thomas; Brabec, Christoph; Walzer, Karsten; Pfeiffer, Martin; Deibel, Carsten (Physical Review Letters, 2023)
The density of states (DOS) is fundamentally important for understanding physical processes in organic disordered semiconductors, yet hard to determine experimentally. We evaluated the DOS by considering recombination via tail states and using the temperature and open-circuit voltage (V_oc) dependence of the ideality factor. By performing Suns-V_oc measurements, we find that the energetic disorder increases deeper into the band gap, which is not expected for a Gaussian or exponential DOS. The linear dependence of the disorder on energy reveals the power-law DOS in organic solar cells.
CRIS-Team Scopus Importer:2023-07-07

Practical limits of multijunction solar cells - Peters, Ian Marius; Rodríguez Gallegos, Carlos David; Lüer, Larry; Hauch, Jens; Brabec, Christoph J. (Progress in Photovoltaics, 2023)
Multijunction solar cells offer a path to very high conversion efficiency, exceeding 60% in theory. Under ideal conditions, efficiency increases monotonically with the number of junctions. In this study, we explore technical and economic mechanisms acting on tandem solar cells. We find that these mechanisms produce limitations that are the more pronounced the greater the number of junction is and, hence, limit the ideal number of junctions, as well as the corresponding efficiencies. Spectral variations induce current losses in series-connected tandem solar cells. For Denver, we find that these losses reduce achievable harvesting efficiencies to 51% for non-concentrated light and that they restrict the ideal number of junctions to less than nine. Independently operated solar cells suffer from optical losses with similar consequences. Even high optical efficiencies of 99% restrict the ideal number of junctions to below 10 and reduce achievable efficiencies by more than 10%. Only architectures with a sequential cell illumination are more resilient to these losses. Restricting available materials reveals that a sufficiently low band gap for the bottom cell of 0.9 eV or below is expedient to realize high efficiencies. Economic considerations show that five junctions or less are economically ideal for most conceivable applications.
CRIS-Team Scopus Importer:2023-05-26
Keywords: multijunction solar cell; solar spectrum; techno-economic modelling

Precise control of process parameters for >23% efficiency perovskite solar cells in ambient air using an automated device acceleration platform - Zhang, Jiyun; Wu, Jianchang; Barabash, Anastasia; Du, Tian; Qiu, Shudi; Le Corre, Vincent Marc; Zhao, Yicheng; Zhang, Kaicheng; Schmitt, Frederik; Peng, Zijian; Tian, Jingjing; Li, Chaohui; et al. (Energy and Environmental Science, 2024)
Achieving high-performance perovskite photovoltaics, especially in ambient air, is critically dependent on the precise optimization of process parameters. However, traditional manual methods often struggle to effectively control the key variables. This inherent challenge requires a paradigm shift toward automated platforms capable of precise and reproducible experiments. Herein, we use a fully automated device acceleration platform (DAP) to optimize air-processed parameters for preparing perovskite devices using a two-step sequential deposition technique. Over ten process parameters with significant potential to influence device performance are systematically optimized. Specifically, we delve into the impact of the dripping speed of organic ammonium halide, a parameter that is difficult to control manually, on both perovskite film and device performance. Through the targeted design of experiments, we reveal that the dripping speed significantly affects device performance primarily by adjusting the residual PbI2 content in the films. We find that optimal dripping speeds, such as 50 µL s−1, contribute to top-performance devices. Conversely, excessively fast or slow speeds result in devices with comparatively poorer performance and lower reproducibility. The optimized parameter set enables us to establish a standard operation procedure (SOP) for additive-free perovskite processing in ambient conditions, which yield devices with efficiencies surpassing 23%, satisfactory reproducibility, and state-of-the-art photo-thermal stability. This research underscores the importance of understanding the causality of process parameters in enhancing perovskite photovoltaic performance. Furthermore, our study highlights the pivotal role of automated platforms in discovering innovative workflows and accelerating the development of high-performing perovskite photovoltaic technologies.
CRIS-Team Scopus Importer:2024-07-19

Precursor-Engineered Volatile Inks Enable Reliable Blade-Coating of Cesium–Formamidinium Perovskites Toward Fully Printed Solar Modules - Du, Tian; Rehm, Viktor; Qiu, Shudi; Pal, Subhajit; Jang, Dongju; Peng, Zijian; Zhang, Jiyun; Yuan, Haozhen; Briscoe, Joe; Heiß, Wolfgang; Brabec, Christoph; Egelhaaf, Hans-Joachim (Advanced Science, 2024)
Reliable fabrication of large-area perovskite films with antisolvent-free printing techniques requires high-volatility solvents, such as 2-methoxyethanol (2ME), to formulate precursor inks. However, the fabrication of high-quality cesium–formamidinium (Cs–FA) perovskites has been hampered using volatile solvents due to their poor coordination with the perovskite precursors. Here, this issue is resolved by re-formulating a 2ME-based Cs0.05FA0.95PbI3 ink using pre-synthesized single crystals as the precursor instead of the conventional mixture of raw powders. The key to obtaining high-quality Cs–FA films lies in the removal of colloidal particles from the ink and hence the suppression of colloid-induced heterogeneous nucleation, which kinetically facilitates the growth of as-formed crystals toward larger grains and improved film crystallinity. Employing the precursor-engineered volatile ink in the vacuum-free, fully printing processing of solar cells (with carbon electrode), a power conversion efficiency (PCE) of 19.3%, a T80 (80% of initial PCE) of 1000 h in ISOS-L-2I (85 °C/1 Sun) aging test and a substantially reduced bill of materials are obtained. The reliable coating methodology ultimately enables the fabrication of carbon-electrode mini solar modules with a stabilized PCE of 16.2% (average 15.6%) representing the record value among the fully printed counterparts and a key milestone toward meeting the objectives for a scalable photovoltaic technology.
CRIS-Team Scopus Importer:2024-05-24
Keywords: cesium formamidinium perovskite; reliable blade coating; single crystals; solar module; volatile ink

Predicting layer thicknesses by numerical simulation for meniscus-guided coating of organic photovoltaics - Gumpert, Fabian; Janßen, Annika; Brabec, Christoph; Egelhaaf, Hans-Joachim; Lohbreier, Jan; Distler, Andreas (Engineering Applications of Computational Fluid Mechanics, 2023)
To achieve maximum efficiency in organic photovoltaics (OPV), functional layers with uniform and exactly predefined thickness are required. An in-depth understanding of the coating process is therefore crucial for an accurate process control. In this paper, the meniscus-guided blade coating process, which is the most commonly used process for the manufacturing of organic electronics, is investigated by experimental and numerical methods. A computational fluid dynamics (CFD) model is created to simulate the coating behaviour of P3HT:O IDTBR, an industrial state-of-the-art active material system used in OPV, and its results' independence of numerical parameters is ensured. In particular, the influence of the coating velocity and the initially injected fluid volume on the resulting wet film thickness is studied. The developed CFD analysis is able to reproduce the experimental results with very high accuracy. It is found that the film thickness follows a power law dependence on the velocity (˜v 2/3) and a linear dependence on the ink volume (˜V). Accordingly, an analytical expression based on our theoretical considerations is presented, which predicts the wet film thickness as a function of the coating velocity and the ink volume only based on easily accessible ink properties. Consequently, this CFD model can effectively substitute time-consuming and expensive experiments, which currently have to be performed manually in the laboratory for a multitude of novel material systems, and thus supports highly accelerated material research. Moreover, the results of this work can be used to achieve homogeneous large-area coatings by utilising accelerated blade coating.
CRIS-Team Scopus Importer:2023-08-18
Keywords: coating techniques; COMSOL multiphysics; Numerical simulation; organic photovoltaics; renewable energy; thin films

Preface - Sablon, Kimberly A.; Heier, Jakob; Tatavarti, Sudersena Rao; Olson, Dana C.; Kippelen, Bernard; Fu, Lan; Brabec, Christoph; Wang, Zhiming; Nüesch, Frank A. (Materials Research Society Symposium Proceedings, 2013)

Preface to selected papers from EMRS 2011 Symposium Q: Engineering of wide bandgap semiconductor materials for energy saving Preface - Wellmann, Peter; Syväjärvi, Mikael; Kneissel, Michael; Wang, Rongmin (Thin Solid Films, 2012)
Keywords: wide bandgap semiconductor materials

Preface to the Special Issue of ChemSusChem on Advanced Organic Solar Cells - Brabec, Christoph; Heeney, Martin; Kim, Youngkyoo; Luscombe, Christine K. (Chemsuschem, 2021)
Organic solar cells have garnered much interest as an Earth-abundant and low-energy-production renewable energy source. In their Editorial to the Special Issue on Advanced Organic Solar Cells, Guest Editors Christoph Brabec, Martin Heeney, Youngkyoo Kim, and Christine Luscombe introduce this exciting field and discuss some of the Special Issue's contents.
CRIS-Team Scopus Importer:2021-08-27
Keywords: donor-acceptor systems; nonfullerene acceptors; polymers; renewable energy; solar cells

Prevention of Bunched Basal Plane Dislocation Arrays in 4H-SiC PVT-Growth - Steiner, Johannes; Nguyen, B. D.; Sandfeld, S.; Wellmann, Peter (Trans Tech Publications Ltd, 2023)
To prevent arrays of basal plane dislocations (BPD) forming during grown 4H-SiC single crystals, the growth cell in physical vapor transport (PVT) growth was modified by adapting the temperature gradients, the seed attachment method and the seeding phase. The resulting reduction in stress was modeled numerically and the crystals were investigated by X-ray topography (XRT) and molten potassium hydroxide (KOH) etching. Due to these modifications, the formation of BPD arrays was completely suppressed.
CRIS-Team Scopus Importer:2023-06-23
Keywords: basal plane dislocations; numerical modeling; physical vapor transport; stress; X-ray topography

Printable Dielectric Mirrors with Easily Adjustable and Well-Defined Reflection Maxima for Semitransparent Organic Solar Cells - Bronnbauer, Carina; Hornich, Julian; Gasparini, Nicola; Guo, Fei; Hartmeier, Benjamin; Luechinger, Norman A.; Pflaum, Christoph; Brabec, Christoph; Forberich, Karen (Advanced Optical Materials, 2015)
Building integrated semitransparent thin-film solar cells is a strategy for future eco-friendly power generation. Organic photovoltaics in combination with dielectric mirrors (DMs) are a potential candidate as they promise high efficiencies in parallel to the possibility to adjust the color and thus the transparency of the whole device. A fully solution processed and printable DM with an easily adjustable reflection maximum is presented that can be facilely attached to solar cells. The DM is optimized via optical simulations to the particular needs of the device with regard to photocurrent enhancement. The excellent agreement between experimental and theoretical results confirms the high optical quality of the printed layers with respect to homogeneity and surface roughness. The used inks are organic-inorganic nanocomposites with a large refractive index contrast of ≈0.7. The short-circuit current is enhanced by up to ≈24% for a semitransparent polymer solar cell.
Keywords: Dielectric mirrors; Organic photovoltaics; Printing; Refractive indices; Semitransparent

Printed semi-transparent large area organic photovoltaic modules with power conversion efficiencies of close to 5 % - Lucera, L.; Machui, F.; Schmidt, H. D.; Ahmad, T.; Kubis, P.; Strohm, S.; Hepp, J.; Vetter, Andreas; Egelhaaf, H. -J.; Brabec, Christoph (Organic Electronics, 2017)
Currently, certified lab scale organic photovoltaic (OPV) cells reach efficiencies of more than 12% and life times of 10 years. For commercialization, it is necessary to understand which performance can be reached in fully printed large scale products. Our investigations show that large area, semi-transparent organic photovoltaic modules based on industrially available materials can achieve power conversion efficiencies of more than 4.8% on rigid substrates and 4.3% on flexible ones. The modules processed with a combination of large area coating and laser patterning with an active area of 68.76 cmfor flexible modules and a total area of 197.4 cmfor glass modules offer exceptionally high geometric fill factors of more than 94% and a transparency of more than 10%. The processing recipe and the layout of the modules are based on indications of optical and electrical simulations which allow to produce devices with only negligible losses in comparison to small single cell devices. Losses due to imperfect coating or patterning are identified by thermal imaging.
Keywords: Large area processing; Module; Organic photovoltaics; Renewable energy; Semi-transparent; Slot-die coating

Printed Smart Photovoltaic Window Integrated with an Energy-Saving Thermochromic Layer - Guo, Fei; Chen, Shi; Chen, Zhang; Luo, Hongjie; Gao, Yanfeng; Przybilla, Thomas; Spiecker, Erdmann; Osvet, Andres; Forberich, Karen; Brabec, Christoph (Advanced Optical Materials, 2015)
A smart photovoltaic window is designed and constructed by solution-processing a layer of thermochromic VO2 nanoparticles on top of a semitransparent organic solar cell. The prepared smart window not only produces electricity using the visible part of the solar spectrum but also saves energy via intelligently modulating the amount of NIR radiation passing through the device in response to ambient temperature.

Printing high performance reflective electrodes for organic solar cells - Krantz, Johannes; Forberich, Karen; Kubis, Peter; Machui, Florian; Min, Jie; Stubhan, Tobias; Brabec, Christoph (Organic Electronics, 2015)
In this paper we solve a long term material problem of thin film organic electronics, namely the solution processing of an opaque electrode. Solution processing of opaque metallic top electrodes typically leads to severe shunting problems. We solve this issue by reversing the electrode sequence and print a highly conductive but opaque bottom electrode from metallic precursors. Devices based on these printed bottom electrodes are compared to reference stacks based on evaporated silver. The transparent top electrode is solution processed from silver nanowire inks, which results in highly transparent electrodes with excellent conductivity. The optical, mainly reflective properties of the opaque silver electrode are investigated in comparison to screen-printed silver pastes. The outstanding smoothness of the printed Ag electrode results in high reflectivity and poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based solar cells with η >. 2.5% and high fill factors performing on par with reference devices on evaporated silver electrode layers.
Keywords: Highly reflective electrode; Printed silver precursor ink; Solution processable electrode; Substrate architecture; Superstrate architecture

Printing High-Quality Formanidinum Lead Triiodide Films: Understanding the Critical Role of α-Phase Nucleation Before Thermal Annealing - Qiu, Shudi; Dong, Lirong; Jang, Dongju; Yang, Fu; García Cerrillo, José; Li, Chaohui; Xie, Zhiqiang; Luo, Junsheng; Guo, Fei; Distler, Andreas; Du, Tian; Brabec, Christoph; et al. (Advanced Energy Materials, 2024)
Upscaling the coating of formamidinium lead triiodide (FAPbI3) thin film is essential to realizing full printing and thereby roll-to-roll production of perovskite photovoltaics. However, thin-film FAPbI3 processed from antisolvent-free, printing methods suffer from undesirable degradation to the non-photoactive phase, particularly in ambient processing conditions. Here, the most critical stage in thin-film processing is identified as the gas-quenching treatment of the as-cast wet film. It is crucial to achieve both the nucleation of α-phase nanocrystals and their sufficient growth at room temperature, which are indispensable for templating the growth of a highly crystalline and stable α-FAPbI3 film during subsequent thermal annealing. The gas-quenching-treated film without these α-phase nanocrystals can only be converted to meta-stable α-FAPbI3 by thermal annealing. By precisely identifying the spontaneous doping of cesium ion (Cs+) as a key factor in triggering α-FAPbI3 nucleation and the role of chloride ion (Cl−) in enhancing nanocrystal growth, a generalized mechanism for additive engineering for the precursor ink is theorized. The identified mechanism translates to a power conversion efficiency of 19.36% for fully printed carbon-electrode solar cells, 16.23% for carbon-electrode minimodules, and excellent operational stability, showing the promising potential of this proposed fabrication route in the upscaling of perovskite photovoltaics.
CRIS-Team Scopus Importer:2024-10-25
Keywords: carbon-electrode; cesium ion; FAPbI; nucleation; perovskite photovoltaics

Printing of Large-Scale, Flexible, Long-Term Stable Dielectric Mirrors with Suppressed Side Interferences - Bronnbauer, Carina; Riecke, Arne; Adler, Marius; Hornich, Julian; Schunk, Gerhard; Brabec, Christoph; Forberich, Karen (Advanced Optical Materials, 2017)
Dielectric mirrors are wavelength-selective mirrors which are based on thin film interference effects. Their optical band can precisely be adjusted in width, position, and reflectance by the refractive index of the applied materials, the layers’ thicknesses, and the amount of deposited layers. Nowadays, they are a well-known light management tool for efficiency enhancement in, for example, semitransparent organic solar cells (OSCs) and light guiding in organic light-emitting diodes (OLEDs). However, most of the dielectric mirrors are still fabricated by lab-scale techniques such as spin-coating or physical vapor deposition under vacuum. Large-scale, fully printed (maximum 20 × 20 cm2) dielectric mirrors with adjustable reflectance characteristics are fabricated, using temperatures of maximum 50 °C and alcohol-based inks. According to the moderate processing conditions they can be easily deposited not only on rigid glass substrates but also on flexible foils. They show high stability against humidity, light irradiation, and temperature, positioning themselves as good candidates for applications in OLEDs and OSCs. Eventually, by simulations and experiments it is verified that a moderate degree of variations in layer thickness and surface roughness can suppress side interference fringes, while not impacting the main transmittance minimum or the main reflection maximum, respectively.
Keywords: Bragg reflectors; Dielectric mirrors; Fully printed; Large scale; Lifetime

Processing and Characterization of Vacuum-Free CuInSe2 Thin Films from Nanoparticle-Precursors using Novel Temperature Treatment Techniques - Schuster, Matthias; Sisterhenn, Philipp; Graf, Lorenz; Wellmann, Peter (International Journal of Nanoparticle Research, 2018)
The objective of this work is the processing and characterization of a dense CuInSe2 solar-cell-absorber-layer based on nanoparticulate precursors. Bimetallic copper-indium- and elemental selenium-nanoparticles were synthesized by wetchemical processes and then dispersed in organic solvents as nano-inks. These inks were then printed into different layerstacks on a molybdenum coated float-glass-substrate via doctorblading. The temperature treatment to transform these layerstacks into dense CISe thin films was investigated, using a faceto- face technique and mechanically applied pressure or the repetition of coating and annealing. All absorber layers were characterized with SEM, EDX and XRD. Dense, coarse grained CuInSe2 layers with a thickness ≈ 7 μm were formed and the application of mechanical pressure shows potential to reduce thickness and sinter together the nanoparticles to large grains of ca. 3 μm in size. The face-to-face-annealing ensured keeping a stoichiometric ratio of (Cu+In) / Se ≈ 1, and can help reducing the content of oxides, even when the annealing is performed in ambient atmosphere. With a repetition of coating and annealing, dense CISe layers could be produced at low temperature of only 350 °C.
Keywords: CuInSe2; solar cell absorber; nanoparticles; vacuumfree; face-to-face annealing; uniaxial pressure

Production and Characterization of Tb3+/Yb3+ Co-Activated AlON Thin Films for Down Conversion Applications in Photovoltaic Cells - Tucto, K.; Flores, L.; Guerra, J.; Tofflinger, J.; Dulanto, J.; Grieseler, Rolf; Osvet, Andres; Batentschuk, Miroslaw; Weingartner, R. (MRS Advances, 2017)
Terbium and ytterbium co-doped aluminum oxynitride thin films were grown onto silicon substrates using radiofrequency magnetron sputtering. Aluminum oxynitride samples doped with 4.6 at. % of Yb3+ and co-doped with 0.4 at. % of Tb3+ were obtained. The prepared samples were annealed from 150ºC to 850ºC in steps of 100ºC. By using energy dispersive X-ray analysis we measured the sample composition and the doping concentration. The emission intensities at different annealing temperatures were characterized using photoluminescence measurements upon excitation at 325 nm. The 5D4  7F5 main transition of Tb3+ and the characteristic near infrared emission at 980 nm of Yb3+ were recorded. In order to study the luminescence behavior of the samples in terms of a down conversion process, we have plotted the integrated areas of the main transition peaks versus the annealing temperature.<br
Keywords: lanthanide; luminescence; sputtering

Profitability and LCOE of Small Solar Battery Systemsthe German Case. (Smart Grid Solar - A Bavarian Smart Energy Project) - Stegner, Christoph; Luchscheider, Philipp; Bogenrieder, Josef; German, Reinhard; Brabec, Christoph (Proceedings of the 31st European Photovoltaic Solar Energy Conference (EU PVSEC 2015), 2015)
UnivIS-Import:2016-06-01:Pub.2015.tech.IMMD.IMMD7.profit

Progress and limits of the numerical simulation of SiC bulk and epitaxy growth processes - Pons, Michel; Blanquet, E.; Dedulle, J.M.; Ucar, M.; Wellmann, Peter; Danielsson, Ö.; Ferret, P.; Di Cioccio, L.; Baillet, F.; Chaussende, D.; Madar, R. (Materials Science Forum, 2005)
Modeling and simulation of the SiC growth process is sufficiently mature to be used as a training tool for engineers as well as a decision making tool, e.g. when building new process equipment or up-scaling old ones. It is possible to simulate accurately temperature and deposition distributions, as well as doping. The key of success would be the combined use of simulation, experiments and characterization in a "daily interaction". The main limitation in SiC growth modeling is the accurate knowledge of physical, thermal, radiative, chemical and electrical data for the different components of the reactor. This is the weakest link in developing completely predictive models. In addition, the link between the thermochemical history of the grown material and its structure and defects still needs further development and input of experimental dat
Keywords: SiC growth; sublimation; bulk; CVD; epitaxy; modeling; simulation

Progress in Poly (3-Hexylthiophene) Organic Solar Cells and the Influence of Its Molecular Weight on Device Performance - Wadsworth, Andrew; Hamid, Zeinab; Bidwell, Matthew; Ashraf, Raja S.; Khan, Jafar I.; Anjum, Dalaver H.; Cendra, Camila; Yan, Jun; Rezasoltani, Elham; Guilbert, Anne A. Y.; Azzouzi, Mohammed; Gasparini, Nicola; et al. (Advanced Energy Materials, 2018)
Poly (3-hexylthiophene) (P3HT) was an early frontrunner in the development of donor polymers to be used in organic photovoltaics. A relatively straightforward and inexpensive synthesis suggests that it may be the most viable donor polymer to use in large-scale commercial organic solar cells. Replacing fullerenes with new electron acceptors has led to significant improvements in device performance and stability, with devices now able to exceed an efficiency of 7%. Past studies have reported a dependence of device performance on the molecular weight of the polymer in fullerene-containing blends, however, with nonfullerene acceptors now showing promise a similar study was needed. P3HT blends, with two nonfullerene acceptors (O-IDTBR and EH-IDTBR), were probed using a number of polymer batches with varying molecular weights. O-IDTBR was shown to exhibit a dependence on the polymer molecular weight, with optimal performance achieved with a 34 kDa polymer, while EH-IDTBR displayed an independence in performance with varying polymer molecular weight. Probing the thermal and morphological behavior of the P3HT:O-IDTBR blends suggests that an optimal morphology with pronounced donor and acceptor domains was only achieved with the 34 kDa polymer, and a greater degree of mixing was exhibited in the other blends, likely leading to poorer device performance.
Keywords: IDTBR; molecular weight; nonfullerene; organic photovoltaics; poly (3-hexylthiophene)

Progress of the key materials for organic solar cells - Tong, Yang; Xiao, Zuo; Du, Xiaoyan; Zuo, Chuantian; Li, Yuelong; Lv, Menglan; Yuan, Yongbo; Yi, Chenyi; Hao, Feng; Hua, Yong; Lei, Ting; Lin, Qianqian; et al. (Science China-Chemistry, 2020)
Organic solar cells have attracted academic and industrial interests due to the advantages like lightweight, flexibility and roll-to-roll fabrication. Nowadays, 18% power conversion efficiency has been achieved in the state-of-the-art organic solar cells. The recent rapid progress in organic solar cells relies on the continuously emerging new materials and device fabrication technologies, and the deep understanding on film morphology, molecular packing and device physics. Donor and acceptor materials are the key materials for organic solar cells since they determine the device performance. The past 25 years have witnessed an odyssey in developing high-performance donors and acceptors. In this review, we focus on those star materials and milestone work, and introduce the molecular structure evolution of key materials. These key materials include homopolymer donors, D-A copolymer donors, A-D-A small molecular donors, fullerene acceptors and nonfullerene acceptors. At last, we outlook the challenges and very important directions in key materials development.
CRIS-Team WoS Importer:2020-04-21

Progress of the key materials for organic solar cells 有机太阳电池关键材料研究进展 - Tong, Yang; Xiao, Zuo; Du, Xiaoyan; Zuo, Chuantian; Li, Yuelong; Lv, Menglan; Yuan, Yongbo; Yi, Chenyi; Hao, Feng; Hua, Yong; Lei, Ting; Lin, Qianqian; et al. (SCIENTIA SINICA Chimica, 2020)
Organic solar cells have attracted academic and industrial interests due to the advantages like lightweight, flexibility and roll-to-roll fabrication. Nowadays, 18% power conversion efficiency has been achieved in the state-of-the-art organic solar cells. The recent rapid progress in organic solar cells relies on the continuously emerging new materials, device fabrication technologies, and the deep understanding on film morphology, molecular packing and device physics. Donor and acceptor materials are the key materials for organic solar cells since they determine the device performance. The past 25 years have witnessed an odyssey in developing high-performance donors and acceptors. In this review, we focus on those star materials and milestone work, and introduce the molecular structure evolution of the key materials. These key materials include homopolymer donors, D-A copolymer donors, A-D-A small molecule donors, fullerene acceptors and nonfullerene acceptors. At last, we outlook the challenges and very important directions in key materials development.
CRIS-Team Scopus Importer:2020-10-30
Keywords: D-A copolymer donors; Fullerene acceptors; Key materials; Nonfullerene acceptors; Organic solar cells

Progress on Numerical Reactive Diffusion Modeling of CuInSe2 Phase Formation for Solar Cell Applications - Zweschke, André; Wellmann, Peter (Energy Procedia, 2015)
Numerical modeling of chalcopyrite (CuInSe2) absorber layer formation by rapid thermal annealed stacked elemental layers provides a valuable tool to address the microscopic phase formation process, which significantly determines the absorber layer properties in solar cell applications. We present a one dimensional kinetic modeling tool describing the diffusive reaction kinetics for layers of partial miscible systems. The diffusion is described temperature dependent and by calculating the latent heat the results were compared with differential scanning calorimetry measurement curves. Applying the numerical simulation to elemental layers of Cu, In and Se, first binary compounds are observed. With the presented progress in the model the absorber can completely react to CuInSe2, after sufficient intermixing of the elements. The results show a good agreement with experimental data with regard to the temperature range of the reactions taking place.
Keywords: Chalcopyrite; Reactive Diffusion; Diffusion modeling

Prospects of bulk growth of 3C-SiC using sublimation growth - Wellmann, Peter; Schuh, Philipp; Kollmuß, Manuel; Schöler, Michael; Steiner, Johannes; Zielinski, M.; Mauceri, M.; La Via, F. (Materials Science Forum, 2020)
Free standing 3C-SiC wafers with a dimeter of 50 mm and a thickness of ca. 0.8 mm have been grown on a regular base using 3C-SiC CVD seed transfer from Si wafers to a poly-SiC-carrier and a sublimation epitaxy configuration. Up to the thickness of almost 1 mm, stable growth conditions of the cubic polytype have been achieved. The high supersaturation was kept stable by the proper design of the hot zone that enables a high axial temperature gradient at the growth interface. The Si-rich gas phase was realized by the application of a Tantalum getter that was integrated into the graphite-based growth cell. Furthermore, an adaption of the growth setup allowed the growth of 3C material with a diameter of 95 mm and bulk material up to 3 mm on 25 mm diameter. Computer simulations were used to determine the supersaturation of the growth setup for different source-to-seed distances. The minimum supersaturation necessary for stable growth of cubic SiC was found to be higher 0.1 for seeds already containing the required 3C polytype.
CRIS-Team Scopus Importer:2020-09-04
Keywords: 3C-SiC; Bulk; Large-area; Sublimation epitaxy; Supersaturation

Prospects of nanoscience with nanocrystals - Kovalenko, Maksym V.; Manna, Liberato; Cabot, Andreu; Hens, Zeger; Talapin, Dmitri V.; Kagan, Cherie R.; Klimov, Victor I.; Rogach, Andrey L.; Reiss, Peter; Milliron, Delia J.; Guyot-Sionnnest, Philippe; Konstantatos, Gerasimos; et al. (ACS nano, 2015)
Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today's strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.
Keywords: Characterization; Electronic properties; Imaging techniques; Infrared radiation; Inorganic compounds; Light emission; Nanocrystals; Optoelectronic devices; Semiconductor doping; Surface chemistry Chemical transformations; Colloidal nanocrystals; Compositional modulation; Crystalline nanoparticles; In-situ characterization; Light emitting devices; Narrow size distributions; Self organizations; Semiconductor materials; nanoparticle; quantum dot

Pushing efficiency limits for semitransparent perovskite solar cells - Ramírez Quiroz, César Omar; Levchuk, Ievgen; Bronnbauer, Carina; Salvador, Michael Filipe; Forberich, Karen; Heumüller, Thomas; Hou, Yi; Schweizer, Peter; Spiecker, Erdmann; Brabec, Christoph (Journal of Materials Chemistry A, 2015)
While perovskite-based semitransparent solar cells deliver competitive levels of transparency and efficiency to be envisioned for urban infrastructures, the complexity and sensitivity of their processing conditions remain challenging. Here, we introduce two robust protocols for the processing of sub-100 nm perovskite films, allowing fine-tuning of the active layer without compromising the crystallinity and quality of the semiconductor. Specifically, we demonstrate that a method based on solvent-induced crystallization with a rapid drying step affords perovskite solar cells with 37% average visible transmittance (AVT) and 7.8% PCE. This process enhances crystallization with a preferential phase orientation presumably at the interface, yielding a high fill factor of 72.3%. The second method is based on a solvent-solvent extraction protocol, enabling active layer films as thin as 40 nm and featuring room-temperature crystallization in an ambient environment on a few second time span. As a result, we demonstrate a maximum AVT of 46% with an efficiency of 3.6%, which is the highest combination of efficiency and transparency for a full device stack to date. By combining the two methods presented here we cover a broad range of thicknesses vs. transparency values and confirm that solvent-induced crystallization represents a powerful processing strategy toward high-efficiency semitransparent solar cells. Optical simulations support our experimental findings and provide a global perspective of the opportunities and limitations of semitransparent perovskite photovoltaic devices.

Pushing PbS/Metal‐Halide‐Perovskite Core/Epitaxial‐Ligand‐Shell Nanocrystal Photodetectors beyond 3 µm Wavelength - Killilea, Niall Andrew; Wu, Mingjian; Sytnyk, Mykhailo; Yousefi Amin, Amir-Abbas; Mashkov, Oleksandr; Spiecker, Erdmann; Heiß, Wolfgang (Advanced Functional Materials, 2019)
PbS nanocrystals have been proven to be highly suitable for photodetector fabrication by facile solution processing, and have been successfully tested as photosensitive material in imaging devices. So far, their spectral response has been blue-shifted with respect to that of commercial bulk PbS detectors, due to quantum confinement in nanostructures smaller than the exciton Bohr radius. Here, a PbS nanocrystal synthesis approach is introduced, allowing to surpass this limit, and thus to push the cut-off wavelength to the value of the bulk material. To avoid self-absorbance from ligands within the spectral range of the photoconducting signal, an all inorganic metal-halide-perovskite is applied to form a semiconducting ligand shell. The photoconductors, which are provided from a single drop, do not only show a record in long wavelength operation for PbS nanocrystal detectors but also a room temperature detectivity >1010 Jones, which is on par with that of commercial bulk PbS detectors. Combining these properties might find application in future low-cost infrared imagers, which are currently still elusive due to their high price
Keywords: colloidal nanocrystal; infrared sensing; solution processed perovskites

PV module power prediction by deep learning on electroluminescence images - Assessing the physics learned by a convolutional neural network - Lüer, Larry; Forberich, Karen; Hepp, Johannes; Buerhop-Lutz, Claudia; Winkler, Thilo; Rodrigues, Sandy; Hoffmann, Mathis; Doll, Bernd; Hauch, Jens; Brabec, Christoph; Peters, Ian Marius (Solar Energy Materials and Solar Cells, 2024)
Fast and reliable performance monitoring of photovoltaic modules is essential for economic forecasting in large-scale installations. Deep Learning methods, such as convolutional neural networks, have the potential to predict module power directly from electroluminescence images in an automated workflow. However, neural networks must be trained using large numbers of electroluminescence images of defective modules. Due to budgetary or technical limitations, these training images will always be biased, limiting generalization. Here, we demonstrate a transparent method to discriminate the information learned by a convolutional neural network into generally valid physics and bias. Learning of physics is assessed by providing an artificial, unbiased feature list, which is converted into synthetic electroluminescence images. Using these images, we compare the predictions of the neural network trained on the biased dataset to those of a physics-based equivalent. Bias is assessed by a closer look at the deviations between the predictions from the equivalent circuit model and the trained neural network. The assessment of physics knowledge incorporated by a Deep Learning method gives insight into how the method achieves its predictive capacity.
CRIS-Team Scopus Importer:2023-11-24

PV modules and their backsheets - A case study of a Multi-MW PV power station - Buerhop-Lutz, Claudia; Stroyuk, Oleksandr; Pickel, Tobias; Winkler, Thilo; Hauch, Jens; Peters, Ian Marius (Solar Energy Materials and Solar Cells, 2021)
Degradation of backsheets (BSs) and encapsulant polymer components of silicon PV modules is recognized as one of the main reasons for losses in PV plant performance and lifetime expectations. Here, we report initial insights into the correlation between BS composition of PV-modules and PV power station performance by using a combination of lab- and field-imaging, as well as spectroscopic and electrical characterizations. Using field-suitable near-infrared absorption (NIRA) spectroscopy, the BS structure of 518 PV-modules, 2.5 % of the PV-modules in a 5 MWp PV power station, was identified on-site. The variance of the BS composition was found to be heterogeneous across PV-modules of the same power class from the same manufacturer. Polyamide-based BSs cause in 10 out of 100 inverters ground impedance values below 400 kΩ, which is a typical threshold for inverters connecting to the grid. For fluorinated coating-based BSs this low value is reached 20 times. We conclude these numbers best as possible from the available monitoring data of inverters associated with BS-types from NIRA. The identification of degraded coating-based BSs is challenging, since visually they look healthy and undistinguishable to well performing BSs. The present results demonstrate that a deeper understanding of the relationship between bill-of-materials and performance of PV-modules is necessary to avoid/minimize inverter shut-downs, and that it can be achieved by using combinations of selected field and lab characterization methods with monitoring data.
CRIS-Team Scopus Importer:2021-08-13
Keywords: Backsheets; Coating-based backsheet; Ground impedance; Inverter; Leakage resistance; PV-modules

Qualitative analysis of bulk-heterojunction solar cells without device fabrication: An elegant and contactless method - Baran, Derya; Li, Ning; Breton, Anne-Catherine; Osvet, Andres; Ameri, Tayebeh; Leclerc, Mario; Brabec, Christoph (Journal of the American Chemical Society, 2014)
The enormous synthetic efforts on novel solar cell materials require a reliable and fast technique for the rapid screening of novel donor/acceptor combinations in order to quickly and reliably estimate their optimized parameters. Here, we report the applicability of such a versatile and fast evaluation technique for bulk heterojunction (BHJ) organic photovoltaics (OPV) by utilizing a steady-state photoluminescence (PL) method confirmed by electroluminescence (EL) measurements. A strong relation has been observed between the residual singlet emission and the charge transfer state emission in the blend. Using this relation, a figure of merit (FOM) is defined from photoluminescence and also electroluminescence measurements for qualitative analysis and shown to precisely anticipate the optimized blend parameters of bulk heterojunction films. Photoluminescence allows contactless evaluation of the photoactive layer and can be used to predict the optimized conditions for the best polymer-fullerene combination. Most interestingly, the contactless, PL-based FOM method has the potential to be integrated as a fast and reliable inline tool for quality control and material optimization. © 2014 American Chemical Society.
Keywords: Engineering controlled terms: ElectroluminescencePhotoluminescenceSolar cells Compendex keywords Bulk heterojunction (BHJ)Bulk-heterojunction solar cellsCharge transfer stateFigure of merit (FOM)Material optimizationOptimized conditionsOrganic photovoltaicsSolar cell materials Engineering main heading: Heterojunctions

Qualitative und quantitative Auswertung regelmäßig durchgeführter aIR-Inspektionen von PV-Anlagen mit typischem PID-Muster - Fecher, Frank W.; Buerhop-Lutz, Claudia; Pickel, Tobias; Hundhausen, Martin; Zetzmann, Cornelia; Camus, Christian; Hauch, Jens; Brabec, Christoph (32. Symposium Photovoltaische Solarenergie, 2017)
Potential induzierte Degradation (PID) von Photovoltaik (PV)-Anlagen wird immer öfter an PV-Anlagen diagnostiziert und kann den Ertrag innerhalb weniger Jahre nach der Installation drastisch reduzieren. Als Ursachen werden lange Modulstrings mit hoher Systemspannung von ca. 800 V oder mehr sowie Temperatur und Feuchte im Wechselspiel mit der Verkapselung genannt [1]. Neuste Untersuchungen gehen davon aus, dass grob geschätzt 19 % der PV-Anlagen in Deutschland PID-Probleme haben [2]. Da PID zu einer zeitlich fortscheitenden Verschlechterung der Anlagen führt, ist es wichtig, möglichst frühzeitig PID und dessen Schadensgrad festzustellen, um erfolgreich Gegenmaßnahmen zu ergreifen. Zur Verifikation von PID wird vielfach Elektrolumineszenz (EL) aber auch Infrarot (IR)-Thermographie herangezogen [3, 4]. Da IR-Inspektionen schnell, zuverlässig und kosteneffizient Hinweise auf PID in Solaranlagen ohne Betriebsunterbrechung geben, ist es unser Ziel, eine Methode zur schnellen Diagnose von PID vorzustellen, die gleichzeitig eine quantitative Bestimmung des Schädigungsgrads und regelmäßige Nachmessungen erlaubt. In diesem Artikel stellen wir typische PID-Muster anhand von Infrarotluftaufnahmen (aIR) vor. Die Auswertung dieser zeigte auch erste Zusammenhänge zum Leistungsverlust der PV-Anlage. Des Weiteren wurde das Fortschreiten der PID in den Anlagen regelmäßig sowie der Einsatz von PID-Regenerationsboxen untersucht.

Quality control of polymer solar modules by lock-in thermography - Hoppe, Harald; Bachmann, Jonas; Muhsin, Burhan; Druee, Karl-Heinz; Riedel, Ingo; Gobsch, Gerhard; Buerhop-Lutz, Claudia; Brabec, Christoph; Dyakonov, Vladimir (Journal of Applied Physics, 2010)
We have characterized lateral imperfections of photovoltaic modules based on solution processed polymer-fullerene semiconductor blends by means of lock-in thermography (LIT). The active layer of the solar cell modules is based on the heterogeneous organic semiconductor system poly(3-hexylthiophene):phenyl-C -butyric acid methyl ester and the power conversion efficiency of the modules reached nearly 2% under irradiation of an AM 1.5 solar simulator. Applying highly sensitive LIT allowed us to detect several kinds of laterally distributed defects originating from imperfections in the respective functional layers as well as in the quality of encapsulation. We show that LIT is a powerful method for the quality control of large area polymer solar cells and modules, enabling fast feedback for optimization of production parameters. © 2010 American Institute of Physics.

Quantification of Efficiency Losses Due to Mobile Ions in Perovskite Solar Cells via Fast Hysteresis Measurements - Le Corre, Vincent Marc; Diekmann, Jonas; Peña‐Camargo, Francisco; Thiesbrummel, Jarla; Tokmoldin, Nurlan; Gutierrez-Partida, Emilio; Peters, Karol Pawel; Perdigón-Toro, Lorena; Futscher, Moritz H; Lang, Felix; Warby, Jonathan; Snaith, Henry J.; et al. (Solar RRL, 2022)
Perovskite semiconductors differ from most inorganic and organic semiconductors due to the presence of mobile ions in the material. Although the phenomenon is intensively investigated, important questions such as the exact impact of the mobile ions on the steady-state power conversion efficiency (PCE) and stability remain. Herein, a simple method is proposed to estimate the efficiency loss due to mobile ions via "fast-hysteresis" measurements by preventing the perturbation of mobile ions out of their equilibrium position at fast scan speeds (approximate to 1000 V s(-1)). The "ion-free" PCE is between 1% and 3% higher than the steady-state PCE, demonstrating the importance of ion-induced losses, even in cells with low levels of hysteresis at typical scan speeds (approximate to 100mv s(-1)). The hysteresis over many orders of magnitude in scan speed provides important information on the effective ion diffusion constant from the peak hysteresis position. The fast-hysteresis measurements are corroborated by transient charge extraction and capacitance measurements and numerical simulations, which confirm the experimental findings and provide important insights into the charge carrier dynamics. The proposed method to quantify PCE losses due to field screening induced by mobile ions clarifies several important experimental observations and opens up a large range of future experiments.
Keywords: hysteresis; mobile ions; perovskite solar cells

Quantifying the Absorption Onset in the Quantum Efficiency of Emerging Photovoltaic Devices - Almora, Osbel; Cabrera, Carlos I.; García Cerrillo, José; Kirchartz, Thomas; Rau, Uwe; Brabec, Christoph (Advanced Energy Materials, 2021)
The external quantum efficiency (EQE), also known as incident-photon-to-collected-electron spectra are typically used to access the energy dependent photocurrent losses for photovoltaic devices. The integral over the EQE spectrum results in the theoretical short-circuit current under a given incident illumination spectrum. Additionally, one can also estimate the photovoltaic bandgap energy (Eg) from the inflection point in the absorption threshold region. The latter has recently been implemented in the “Emerging PV reports,” where the highest power conversion efficiencies are listed for different application categories, as a function of Eg. Furthermore, the device performance is put into perspective thereby relating it to the corresponding theoretical limit in the Shockley–Queisser (SQ) model. Here, the evaluation of the EQE spectrum through the sigmoid function is discussed and proven to effectively report the Eg value and the sigmoid wavelength range λs, which quantifies the steepness of the absorption onset. It is also shown how EQE spectra with large λs indicate significant photovoltage losses and present the corresponding implications on the photocurrent SQ model. Similarly, the difference between the photovoltaic and optical bandgap is analyzed in terms of λs.
CRIS-Team Scopus Importer:2021-03-19
Keywords: bandgap energy; emerging photovoltaics; external quantum efficiency; solar cells

Quantifying the Energy Losses in CsPbI2Br Perovskite Solar Cells with an Open-Circuit Voltage of up to 1.45 v - Tian, Jingjing; Zhang, Kaicheng; Xie, Zhiqiang; Peng, Zijian; Zhang, Jiyun; Osvet, Andres; Lüer, Larry; Kirchartz, Thomas; Rau, Uwe; Li, Ning; Brabec, Christoph (ACS Energy Letters, 2022)
CsPbI2Br perovskite solar cells (PSCs) have attracted much interest because of their thermodynamic stability, relatively stable cubic perovskite phase, and their potential as a top cell for tandem applications. However, the open-circuit voltage (VOC) reported to date is in most cases well below the detailed balance (DB) limit for single-junction PSCs. Here, we demonstrate that adding lead acetate to the CsPbI2Br precursor allows us to substantially reduce losses due to nonradiative recombination. Corresponding champion devices reach a power conversion efficiency (-) of 16.7% and a highest VOC value of 1.45 V, which represents 90% of the DB limit for single-junction PSCs at a bandgap of 1.89 eV. In order to disentangle the nonradiative recombination loss mechanisms, we quantify the origin of energy losses by calculating the radiative limit of the open-circuit voltage (VOCrad) and the quasi-Fermi level splitting (QFLS) of perovskite films with and without other functional layers. We further analyze the strategies to reduce the residual losses in order to push the efficiency beyond the 90% theoretical limit.
CRIS-Team Scopus Importer:2022-11-18

Quantifying the extent of contact doping at the interface between high work function electrical contacts and poly(3-hexylthiophene) (P3HT) - Shallcross, R. Clayton; Stubhan, Tobias; Ratcliff, Erin L.; Kahn, Antoine; Brabec, Christoph; Armstrong, Neal R. (Journal of Physical Chemistry Letters, 2015)
We demonstrate new approaches to the characterization of oxidized regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) that results from electronic equilibration with device-relevant high work function electrical contacts using high-resolution X-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy (PES). Careful interpretation of photoemission signals from thiophene sulfur atoms in thin (ca. 20 nm or less) P3HT films provides the ability to uniquely elucidate the products of charge transfer between the polymer and the electrical contact, which is a result of Fermi-level equilibration between the two materials. By comparing high-resolution S 2p core-level spectra to electrochemically oxidized P3HT standards, the extent of the contact doping reaction is quantified, where one in every six thiophene units (ca. 20%) in the first monolayer is oxidized. Finally, angle-resolved XPS of both pure P3HT and its blends with phenyl-C61-butyric acid methyl ester (PCBM) confirms that oxidized P3HT species exist near contacts with work functions greater than ca. 4 eV, providing a means to characterize the interface and "bulk" region of the organic semiconductor in a single film.
Keywords: band bending; contact doping; electrochemistry; P3HT; photoelectron spectroscopy

Quantitative Analysis of Charge Dissociation by Selectively Characterizing Exciton Splitting Efficiencies in Single Component Materials - He, Yakun; Hanisch, Benedict; Osvet, Andres; Lüer, Larry; Aubele, Anna; Bäuerle, Peter; Li, Weiwei; Li, Ning; Brabec, Christoph (Israel Journal of Chemistry, 2021)
Exciton dissociation between donor and acceptor is the decisive process to determine the photovoltaic performance of organic solar cells (OSCs). The rather fast dynamics of photo-induced charge generation in well-optimized bulk heterojunction (BHJ) composites complicate the quantitative analysis of the charge generation efficiency for donor and acceptor units. Herein, we report time-resolved photoluminescence (TRPL) investigations and demonstrate their potential as a quantitative and contactless characterization technique allowing to separately determine the exciton splitting efficiency of donor and acceptor moieties in selected single component materials. We demonstrate that the exciton splitting efficiency for donor or acceptor moieties can be separately adjusted in these materials by post-treatment, while the corresponding BHJ composites undergo excessive phase separation upon external stress. By relating the separate exciton splitting efficiencies to their photovoltaic performance, the limiting factor in corresponding OSCs, either charge generation or recombination, is identified.
CRIS-Team Scopus Importer:2021-10-22
Keywords: charge dissociation; charge transfer; exciton splitting efficiency; organic solar cells; single component materials; transient photoluminescence

Quantitative Analysis of the Separate Influences of Material Composition and Local Defects on the Voc of PV Devices: An Exemplary Study on CIGS - Hepp, Johannes; Vetter, Andreas; Hofbeck, Bernhard; Sultan, Umair; Hauch, Jens; Camus, Christian; Brabec, Christoph (IEEE Journal of Photovoltaics, 2020)
A lot of effort is required in order to close the efficiency gap between laboratory record Cu(In,Ga)Se 2 (CIGS) solar cells and commercially produced modules. An obstacle easily overlooked is the fact that reduced module performance may have multiple root causes. Here, we show how to isolate two main factors that influence the open circuit voltage (Voc) of photovoltaic devices, the material composition and the local defects, here shunts in particular, i.e., regions of significantly lowered resistivity. In order to characterize the material composition, we used electroluminescence spectroscopy. Additionally, we investigated the effect of local defects on the Voc, by the use of thermography. Which of both factors dominates the resulting Voc depends on the insolation intensity under which the device is operated. We established a model in order to estimate the Voc as a function of the luminescence peak wavelength, the quantification of the thermal hot spots caused by local defects, and the insolation intensity. Subsequently, we investigated how these factors affect the electrical conversion efficiency of the device. The combination of these two measurement techniques results in a more conclusive evaluation of the electrical parameters, thereby enabling an improved quality assessment to allow a successive process optimization. This approach is also applicable to other thin film PV technologies such as perovskite and organic solar cells.<br
Keywords: Cu(In; Ga)Se2 (CIGS); infrared (IR) imaging; luminescence; photovoltaic cells; quality management

Quantitative Assessment of the Influence of Camera and Parameter Choice for Outdoor Electroluminescence Investigations of Silicon Photovoltaic Panels - Doll, Bernd; Del Rivero, E. Calderón; Hepp, Johannes; Pickel, Tobias; Buerhop, Claudia; Knecht, R.; Camus, C.; Hauch, Jens; Parisi, J.; Brabec, Christoph (Zeitschrift für Naturforschung Section A - A Journal of Physical Sciences, 2019)
With the spread of photovoltaics (PV) and increasing diversity in PV panel technology, quantitative comparison of the modules is highly desirable for consistent on-site quality assessment. Electroluminescence imaging reveals many defects, such as macroscopic crystal or electrical contact defects, but quantitative comparison outside the laboratory without controlled environment is still difficult, especially for different detector technologies. Here, we show how this problem can be addressed by adding reference spots in the module area: One passive dark spot and an active bright spot composed of a high-power back-contacted silicon PV cell. Those reference spots are used to evaluate the module's electroluminescence signal under different environmental conditions and to establish comparable results. Additionally, the comparison of images acquired with different camera technology detectors, such as silicon and InGaAs, is realised for signal levels.
CRIS-Team Scopus Importer:2019-07-12
Keywords: Defect Detection; Electroluminescence; Outdoor; Photovoltaic; Quantitative Comparison

Quantitative assessment of the power loss of silicon PV modules by IR thermography and its dependence on data-filtering criteria - Teubner, Janine; Buerhop, Claudia; Pickel, Tobias; Hauch, Jens; Camus, Christian; Brabec, Christoph (Progress in Photovoltaics, 2019)
Reliability and quality control of photovoltaic (PV) plants increases in importance as the relevance of PV for the worldwide renewable energy production grows. In this study, a new method is presented which allows for a quantitative assessment of silicon PV module performance solely by relying on the cell temperatures measured via thermography (IR). A module temperature and power key figure are formulated and found to correlate very well with a linear relationship. The dependence of the deduced correlation's precision on measurement conditions is calculated and discussed. It facilitates decision-making because optimal measurement conditions usually occur only very rarely, such that a compromise between data quality and measurement frequency has to be found. The power loss correlation presented in this paper may be used as part of a maintenance routine in order to ensure the best possible long-term performance of the PV plant over its lifetime. The practical application in the field is outlined and explained.
CRIS-Team Scopus Importer:2019-08-23
Keywords: empirical power correlation; infrared measurement; PV; silicon modules; UAV

Quantitative determination of the doping level distribution in n-type GaAs using absorption mapping - Wellmann, Peter; Albrecht, A.; Künecke, Ulrike; Birkmann, Bernhard; Müller, Georg; Jurisch, M. (European Physical Journal-Applied Physics, 2004)
We present an optical technique based on absorption measurements for the determination of the charge carrier concentration and its lateral distribution in n-type doped GaAs wafers. Calibration plots were determined in the charge carrier concentration range of 3x10(17) m(-3)...1x10(18) cm(-3) (range of trust up to 3x10(18) cm(-3)) which is technically relevant for applications of GaAs wafers as substrate for laser and light emitting diodes. The error of the optical technique is in the range of 10%... 15% and is comparable to electrical Hall measurements. The sensitivity of the setup, i.e. smallest detectable variation of doping (and hence charge carrier) concentration, is less than 1% in an area of 5x5 mm(2) and about 20% across the 3 inch area. Absorption mappings of the charge carrier and hence doping homogeneity are presented for n-type GaAs:Si and GaAs:Te.
Keywords: III-V semiconductors; Optical properties of bulk materials and thin films; Semiconductors; charge carrier concentration; absorption; mapping

Quantitative imaging of shunts in organic photovoltaic modules using lock-in thermography - Besold, S.; Hoyer, U.; Bachmann, J.; Swonke, Th.; Schilinsky, P.; Steim, R.; Brabec, Christoph (Solar Energy Materials and Solar Cells, 2014)
Thermographic methods are widely established to detect shunts in photovoltaic technologies. In all methods the cell has to be directly accessible to determine quantitative shunt values. In this manuscript a precise method is presented to determine shunt values in solar modules consisting of series connected bulk-heterojunction organic solar cells without the need to access the single cells separately. It will be shown that, although parallel resistances in organic solar cells show a strong dependence on the illumination intensity, the shunts that are detected using illuminated lock-in-thermography are strongly ohmic and therefore can be assigned a distinct ohm value. (c) 2014 Elsevier B.V. All rights reserved.
Keywords: Organic photovoltaics; Thermography; Simulation; Defect imaging; Parallel resistance

Quantitative Study of Potential Induced Degradation of a Roof-Top PV-Installation with IR-Imaging - Buerhop-Lutz, Claudia; Pickel, Tobias; Fecher, Frank W.; Zetzmann, Cornelia; Hauch, Jens; Camus, Christian; Brabec, Christoph (33rd European Photovoltaic Solar Energy Conference and Exhibition, 2017)
Potential induced degradation (PID) causes severe damage and financial losses even in modern PVinstallations. In Germany, approximately 19% of PV-installations suffer from PID and resulting power loss. This paper focuses on the impact of PID in real installations. The analysis focuses exemplarily on a 314 kWp PV-system in the Atlantic coastal climate. Infrared (IR)-imaging is used for identifying PID without operation interruption. Historic electric performance monitoring data for several years are analyzed on system and string level. Additionally, punctually measured module string current-voltage (I-V)-curves are considered. The data sets are combined for understanding the PID behavior of this PV plant. The number of PID affected cells within a string varies strongly between 1 to 22% with the string position on the building complex. A PID-resistance which correlates to the defined PID-defect-ration is presented. Finally, indicators for the reliable determination of PID is given.
Keywords: potential induced degradation (PID); IR-imaging; PID criteria; power loss; PID-resistance

Quantitative study on the role of supersaturation during sublimation growth on the yield of 50 mm diameter 3C-SiC - Rankl, Dominik; Jokubavicius, Valdas; Syväjärvi, Mikael; Wellmann, Peter (Materials Science Forum, 2015)
We have investigated the crystal growth of 3C-SiC using physical vapor transport growth in the temperature range from 1800°C to 1950°C. The supersaturation was determined using numerical modeling of the temperature field and gas phase composition by applying quasiequilibrium thermodynamic conditions. Analysis of the 3C-SiC yield was carried out by optical microscopy, optical absorption, Raman spectroscopy and x-ray analysis. Quantitative data on supersaturation are compared with most stable 3C-SiC nucleation and growth condition. Our interest is in particular to understand how to use sublimation epitaxy experience in 3C-SiC growth to a PVT set up that could open up the possibility of large area bulk growth. As indicative measure, the application to large area growth in a physical vapor transport growth reactor is briefly addresse
Keywords: Cubic SiC; Bulk growth; Sublimation Growth; Supersaturation

Quantum yield of Eu2+ emission in (Ca1-xSr x)S:Eu light emitting diode converter at 20-420 K - Xia, Qi; Batentschuk, Miroslaw; Osvet, Andres; Winnacker, Albrecht; Schneider, Juergen (Radiation Measurements, 2010)
A series of red-emitting light converters Ca1-xSrxS:Eu2+, with tunable composition-dependent emission maxima were synthesized and characterized concerning their photoluminescent (PL) properties. X-ray diffraction patterns, photoluminescence spectra, luminosities and quantum yields were compared for phosphors with strontium concentrations varying from 0 to 100%. The maxima wavelength of emission shifts from 663 down to 619 nm, originating from the dependence of Eu2+ 5d state energy on the surrounding crystal field. Upon increasing the temperature from 20 to 420 K, a broadening of emission spectra along with thermal quenching of emission intensity and quantum yield occurs. Satisfying PL properties and their thermal stability demonstrate that the phosphors could be used as light converters in light emitting diodes (LEDs).
Keywords: Light emitting diodes; Quantum yield; Phosphors; Temperature stability; Sulfides

Quasi-epitaxial Metal-Halide Perovskite Ligand Shells on PbS Nanocrystals - Sytnyk, Mykhailo; Yakunin, Sergii; Schoefberger, Wolfgang; Lechner, Rainer T.; Burian, Max; Ludescher, Lukas; Killilea, Niall Andrew; Yousefi Amin, Amir-Abbas; Kriegner, Dominik; Stangl, Julian; Groiss, Heiko; Heiß, Wolfgang (Acs Nano, 2017)
Epitaxial growth techniques enable nearly defect free heterostructures with coherent interfaces, which are of utmost importance for high performance electronic devices. While high-vacuum technology-based growth techniques are state-of-the art, here we pursue a purely solution processed approach to obtain nanocrystals with eptaxially coherent and quasi-lattice matched inorganic ligand shells. Octahedral metal-halide clusters, respectively 0-dimensional perovskites, were employed as ligands to match the coordination geometry of the PbS cubic rock-salt lattice. Different clusters (CH3NH3+)((6-x))[M((x+))Hal(6)]((6-x))-(Mx+ = Pb(II), Bi(III), Mn(II), In(III), Hal = Cl, I) were attached to the nanocrystal surfaces via a scalable phase transfer procedure. The ligand attachment and coherence of the formed PbS/ligand core/shell interface was confirmed by combining the results from transmission electron microscopy, small-angle X-ray scattering, nuclear magnetic resonance spectroscopy and powder X-ray diffraction. The lattice mismatch between ligand shell and nanocrystal core plays a key role in performance. In photoconducting devices the best performance (detectivity of 2 X 10(11) cm Hz (1/2)/W with > 110 kHz bandwidth) was obtained with (CH3NH3)(3)BiI6 ligands, providing the smallest relative lattice mismatch of ca. -1%. PbS nanocrystals with such ligands exhibited in millimeter sized bulk samples in the form of pressed pellets a relatively high carrier mobility for nanocrystal solids of similar to 1.3 cm(2)/(V s), a carrier lifetime of similar to 70 mu s, and a low residual carrier concentration of 2.6 X 10(13) cm(-3). Thus, by selection of ligands with appropriate geometry and bond lengths optimized quasi-epitaxial ligand shells were formed on nanocrystals, which are beneficial for applications in optoelectronics.
Keywords: nanocrystals; perovskite; conductive ligands; epitaxy; photodetectors; optoelectronics; semiconductors

Radiation stability of CsBr:Eu needle image plates - Batentschuk, Miroslaw; Neudert, S.; Weidner, Martina; Osvet, Andres; Struye, L.; Tahon, J. -P.; Leblans, P. (Journal of Applied Physics, 2009)
Needle image plates (NIPs) based on CsBr:Eu are a good alternative to the BaFBr:Eu powder image plates due to their higher sensitivity and improved spatial resolution. The x-ray radiation stability of the NIPs produced by Agfa Gevaert was investigated and it was found that the radiation stability of the optimized NIPs is as high as that of the commercial CsI:Tl single crystals and much higher than reported in literature for the CsBr:Eu NIPs. The dependence of the sensitivity of the NIPs on the accumulated dose was determined for three different types of x-ray irradiation. It is shown that degradation of the sensitivity starts at about 10 Gy and it is the strongest for the most hard x-ray beam. If the energy absorbed by a NIP is taken into account, the degradation does not depend on the hardness of the applied x-ray beam. It is suggested that the main reason for the observed high radiation stability of the CsBr:Eu NIPs is the use of the oxygen-free Eu2+-containing precursor CsEuBr3 for the doping in the manufacturing process.

Random Lasing with Systematic Threshold Behavior in Films of CdSe/CdS Core/Thick-Shell Colloidal Quantum Dots - Gollner, Claudia; Ziegler, Johannes; Protesescu, Loredana; Dirin, Dmitry N.; Lechner, Rainer T.; Fritz-Popovski, Gerhard; Sytnyk, Mykhailo; Yakunin, Sergii; Rotter, Stefan; Amin, Amir Abbas Yousefi; Vidal, Cynthia; Hrelescu, Calin; et al. (Acs Nano, 2015)
While over the past years the syntheses of colloidal quantum dots (CQDs) with core/shell structures were continuously improved to obtain highly efficient emission, it has remained a challenge to use them as active materials in laser devices. Here, we report random lasing at room temperature in films of CdSe/CdS CQDs with different core/shell band alignments and extra thick shells. Even though the lasing process is based on random scattering, we find systematic dependencies of the laser thresholds on morphology and laser spot size. To minimize laser thresholds, optimizing the film-forming properties of the CQDs, proven by small-angle X-ray scattering, was found to be more important than the optical parameters of the CQDs, such as biexciton lifetime and binding energy or fluorescence decay time. Furthermore, the observed systematic behavior turned out to be highly reproducible after storing the samples in air for more than 1 year. These highly reproducible systematic dependencies suggest that random lasing experiments are a valuable tool for testing nanocrystal materials, providing a direct and simple feedback for further development of colloidal gain materials toward lasing in continuous wave operation. © 2015 American Chemical Society.
Keywords: exciton-exciton interactions; giant shell quantum dots; plasmonics; random lasing; successive ion layer adsorption and reaction

Rare-Earth Ion-Based Photon Up-Conversion for Transmission-Loss Reduction in Solar Cells - Wang, Hai-Qiao; Osvet, Andres; Batentschuk, Miroslaw; Brabec, Christoph (Rare-Earth Ion-Based Photon Up-Conversion for Transmission-Loss Reduction in Solar Cells, 2021)
Photon up-conversion (UC) describes an anti-Stokes emission process, in which a luminophor emits one higher energy photon after being excited by multiple low-energy photons, among which rare-earth (RE) ion-doped materials present promising UC properties due to unique electron configuration. RE UC materials have been widely studied in solar cells with the purpose to reduce transmission losses, i.e., achieve wide/full solar spectral harvesting and high-power conversion efficiency, by converting unutilized sub-bandgap photons into sensitive resonant photons. This chapter exclusively focuses on RE-doped UC materials and their applications in solar cells. The RE-based UC photophysics, UC enhancement, and applications in solar cells will be reviewed and briefly discussed.<br

Rare-earth ion doped up-conversion materials for photovoltaic applications - Wang, Hao; Batentschuk, Miroslaw; Osvet, Andres; Pinna, Luigi; Brabec, Christoph (Advanced Materials, 2011)
With the aim of utilizing the infrared region of solar radiation to improve solar cell performance, significant progress, including theoretical analysis and experimental achievement, has been made in the field of up-conversion for photovoltaic applications. This Research News article reviews recent progress in the development of rear-earth (RE) ion doped up-conversion materials for solar cell applications. In addition, new trends for RE-ion-doped phosphors are briefly discussed, among them trivalent RE-ion-doped up-conversion materials for organic solar cell applications. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: nonlinear optical materials; phosphors; photovoltaic devices; rare earth ions

Rational Design of High-Performance Wide-Bandgap (≈2 eV) Polymer Semiconductors as Electron Donors in Organic Photovoltaics Exhibiting High Open Circuit Voltages (≈1 V) - Chochos, Christos L.; Katsouras, Athanasios; Gasparini, Nicola; Koulogiannis, Chrysanthos; Ameri, Tayebeh; Brabec, Christoph; Avgeropoulos, Apostolos (Macromolecular Rapid Communications, 2016)
Systematic optimization of the chemical structure of wide-bandgap (≈2.0 eV) "donor-acceptor" copolymers consisting of indacenodithiophene or indacenodithieno[3,2-b]thiophene as the electron-rich unit and thieno[3,4-c]pyrrole-4,6-dione as the electron-deficient moiety in terms of alkyl side chain engineering and distance of the electron-rich and electron-deficient monomers within the repeat unit of the polymer chain results in high-performance electron donor materials for organic photovoltaics. Specifically, preliminary results demonstrate extremely high open circuit voltages (V s) of ≈1.0 V, reasonable short circuit current density (J ) of around 11 mA cm, and moderate fill factors resulting in efficiencies close to 6%. All the devices are fabricated in an inverted architecture with the photoactive layer processed by doctor blade equipment, showing the compatibility with roll-to-roll large-scale manufacturing processes. From the correlation of the chemical structure-optoelectronic properties-photovoltaic performance, a rational guide toward further optimization of the chemical structure in this family of copolymers, has been achieved.
Keywords: High open circuit voltage; Indacenodithiophene; Organic photovoltaics; Ternary solar cells; Wide-bandgap conjugated polymers

Rationale for highly efficient and outdoor-stable terpolymer solar cells - Tang, Hua; Liao, Zhihui; Karuthedath, Safakath; Chen, Si; Liu, Heng; Khan, Jafar I. I.; Babics, Maxime; Yang, Wenchao; Alqurashi, Maryam; He, Yakun; Gorenflot, Julien; Huang, Jiaming; et al. (Energy and Environmental Science, 2023)
Random terpolymerization is an effective approach to achieving highly efficient and outdoor-stable terpolymer photovoltaics. However, the working principle behind this remains unclear. Herein, we report spectroscopic, morphological, and computational results and conclude the previous work on terpolymer research to unveil their structure-property relations and elucidate key rules for high-efficiency and outdoor-stable terpolymer photovoltaics. More specifically, introducing a strong electron-deficient or electron-rich third moiety is suggested to enable broadened absorption with minimized non-radiative voltage losses. Besides, the third component should manipulate the D/A miscibility towards a thermodynamically more stable morphology. However, the content of the third component should be small to maintain molecular orientation and exciton diffusion length in conjunction with optimized phase-separation and crystallinity, in turn reducing bimolecular carrier recombination. Obeying these rules, terpolymer solar cells based on the parent donors D18 and PM6 with enhanced power conversion efficiency (PCE) and excellent outdoor stability are demonstrated. Our findings provide a rationale for explaining and achieving high-performance and outdoor-stable terpolymer photovoltaics, paving the path to commercialization.
CRIS-Team WoS Importer:2023-04-07

Rational Interface Design and Morphology Control for Blade-Coating Efficient Flexible Perovskite Solar Cells with a Record Fill Factor of 81% - Wang, Zhen; Zeng, Linxiang; Zhang, Cuiling; Lu, Yuanlin; Qiu, Shudi; Wang, Chuan; Liu, Chong; Pan, Lijun; Wu, Shaohang; Hu, Jinlong; Liang, Guangxing; Fan, Ping; et al. (Advanced Functional Materials, 2020)
Halide perovskites are one of the ideal photovoltaic materials for constructing flexible solar devices due to relatively high efficiencies for low-temperature solution-processed devices. However, the overwhelming majority of flexible perovskite solar cells are produced using spin coating, which represents a major hurdle for upscaling. Here, a scalable approach is reported to fabricate efficient and robust flexible perovskite solar cells on a polymer substrate. Thiourea is introduced into perovskite precursor solution to modulate the crystal growth, resulting in dense and uniform perovskite thin films on rough surfaces. As a decisive step, a cascade energy alignment is realized for the hole extraction layer by rationally designing a bilayer interface comprised of PEDOT:PSS/PTAA with a distinct offset in the highest occupied molecular orbital levels, enabling markedly enhanced charge extraction and spectral response. An efficiency as high as 19.41% and a record fill factor up to 81% are achieved for flexible perovskite devices processed by a scalable printing method. Equally important, the bilayer interface reinforces the bendability of the indium tin oxide substrate, leading to enhanced mechanical robustness of the flexible devices. These results underpin the importance of morphology control and interface design in constructing high-performance flexible perovskite solar cells.
CRIS-Team Scopus Importer:2020-07-10
Keywords: bilayer interface; blade coating; cascade energy alignment; flexible solar cells; perovskite

Realizing solution-processed monolithic PbS QDs/perovskite tandem solar cells with high UV stability - Zhang, Yannan; Gu, Mengfan; Li, Ning; Xu, Yalong; Ling, Xufeng; Wang, Yongjie; Zhou, Sijie; Li, Fangchao; Yang, Fan; Ji, Kang; Yuan, Jianyu; Ma, Wanli (Journal of Materials Chemistry A, 2018)
Among solution-processed photovoltaic materials, lead sulfide (PbS) colloidal quantum dots (QDs) possess a highly tunable bandgap and strong infrared absorption, while metal halide perovskites show extraordinary external quantum efficiency (EQE) in the visible region, which offers the opportunity to construct an ideal tandem cell of PbS QDs/perovskite. However, it still remains a huge challenge to integrate fragile perovskite films into a monolithic tandem structure. We have for the first time addressed the challenge of preparing stable and efficient tandem solar cells combining PbS colloidal QDs with emerging MAPbI(3) perovskites by all-solution-processing. A perfectly superimposed open circuit voltage and an optimal power conversion efficiency (PCE) of 11.03% have been achieved, which is the highest reported PCE for any PbS QD based tandem solar cell. And we believe this efficiency will be further improved by using perovskites with wider bandgaps to provide more complementary absorption. In addition, the introduction of a PbS subcell can effectively eliminate the hysteresis-effect in metal-halide perovskites, and result in high UV stability and exceptional storage stability under a dry environment without encapsulation. Therefore, we revealed that the synergetic use of QDs and perovskite materials in tandem structures can extend the range of device photo response and simultaneously deliver excellent device performance as well as improved stability. Moreover, our work provided an important starting point for future advances in efficient QDs/perovskite tandem solar cells with more complementary absorption.

Real-time evaluation of thin film drying kinetics using an advanced, multi-probe optical setup - Güldal, Nusret Sena; Kassar, Thaer; Berlinghof, Marvin; Ameri, Tayebeh; Osvet, Andres; Pacios, Roberto; Li Destri, Giovanni; Unruh, Tobias; Brabec, Christoph (Journal of Materials Chemistry C, 2016)
Solution-processed organic photovoltaic devices are advantageous due to their low-cost large area manufacturing techniques, such as slot-die coating, gravure printing and roll-to-roll coating. The final microstructure of a polymer:fullerene bulk-heterojunction (BHJ) film is a fine interplay between solution thermodynamics (e.g. solubility, miscibility...) and kinetics (e.g. solvent evaporation, polymer ordering, phase separation...) during the drying process. In order to design better performing organic photovoltaic devices, gaining knowledge over the drying properties of polymer:fullerene thin films is essential. A novel in situ thin film drying characterization chamber, equipped with white-light reflectometry, laser light scattering and photoluminescence, is presented in combination with grazing-incidence X-ray diffraction on two different polymer:fullerene bulk heterojunctions based on poly-(3-hexylthiophene-2,5-diyl) (P3HT) and polythieno[3,2b]thiophene-diketopyrrolopyrrole-co-thiophene (DPP-TT-T) polymers. With photoluminescence applied for the first time as an in situ method for such drying studies, these single-chamber measurements track the fine interplay between thermodynamics and kinetics of thin film drying and provide invaluable information on solution behavior and microstructure formation.
Keywords: Engineering controlled terms: Coatings; Drying; Fullerenes; Heterojunctions; Kinetics; Light scattering; Microstructure; Phase separation; Photoluminescence; Plastic coatings; Polymer films; Polymers; Rolls (machine components); Solubility; Thermodynamics; Thiophene; X ray diffraction Bulk heterojunction (BHJ); Grazing incidence X-ray diffraction; Manufacturing techniques; Microstructure formation; Organic photovoltaic devices; Poly(3-hexylthiophene-2; 5-diyl); Solution thermodynamics; Thermodynamics and kinetics Engineering main heading: Thin films

Real-time in-situ observation of morphological changes in organic bulk-heterojunction solar cells by means of capacitance measurements - Lungenschmied, Christoph; Bauer, Siegfried; Schwoediauer, Reinhard; Rodman, Sheila; Fournier, Darcy; Dennler, Gilles; Brabec, Christoph (Journal of Applied Physics, 2011)
We present a method to directly study temperature induced structural and morphological changes in the active layer of organic solar cells by capacitance determination. At sufficiently high frequencies the change in device capacitance with temperature is dominated by the expansion of the organic layer. Variations in the expansion coefficient or the permittivity are visible in the capacitive response. We have studied thin film devices based on poly(3- hexyl)thiophene (P3HT) as well as organic solar cells made from blends of P3HT with a fullerene derivative. Temperature induced transitions are correlated to structural changes in the polymer (e.g., glass transition) as well as morphological changes of the blend; the morphology transition driven by phase separation sets in around 120 C. The advantage of our capacitance method is substantiated as we have demonstrated that confinement due to the substrate and the top electrode alters the effect of temperature on thin films as compared to the bulk material. © 2011 American Institute of Physics.
Keywords: Active Layer; Bulk materials; Capacitance method; Capacitive response; Device capacitance; Effect of temperature; Expansion coefficients; Fullerene derivative; High frequency; In-situ observations; Morphological changes; Morphology transitions; Organic bulk-heterojunction solar cells; Organic layers; Organic solar cell; Structural change; Temperature-induced Engineering controlled terms: Capacitance; Capacitance measurement; Glass transition; Heterojunctions; Morphology; Phase separation; Polymer films; Thin film devices; Thin films; Thiophene Engineering main heading: Solar cells

Real-Time Investigation of Intercalation and Structure Evolution in Printed Polymer:Fullerene Bulk Heterojunction Thin Films - Kassar, Thaer; Güldal, Nusret Sena; Berlinghof, Marvin; Ameri, Tayebeh; Kratzer, Andreas; Schroeder, Bob C.; Li Destri, Giovanni; Hirsch, Andreas; Heeney, Martin; Mcculloch, Iain; Brabec, Christoph; Unruh, Tobias (Advanced Energy Materials, 2016)
The complex intermixing morphology is critical for the performance of the nanostructured polymer:fullerene bulk heterojunction (BHJ) solar cells. Here, time resolved in situ grazing incidence X-ray diffraction and grazing incidence small angle X-ray scattering are used to track the structure formation of BHJ thin films formed from the donor polymer poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) with different fullerene derivative acceptors. The formation of stable bimolecular crystals through the intercalation of fullerene molecules between the side chains of polymer crystallites is investigated. Such systems exhibit more efficient exciton dissociation but lower photo-conductance and faster decay of charges. On the basis of the experimental observations, intercalation obviously takes place before or with the formation of the crystalline polymer domains. It results in more stable structures whose volume remains constant upon further drying. Three distinct periods of drying are observed and the formation of unidimensional fullerene channels along the π-stacking direction of polymer crystallites is confirmed.
Keywords: fullerenes; grazing incidence diffraction; intercalation dynamics; organic solar cells; thin films

Real-Time Investigations on the Formation of CuIn(S,Se)(2) While Annealing Precursors With Varying Sulfur Content - Hölzing, Astrid; Schurr, Roland; Jost, Stefan; Palm, Jörg; Deseler, Klaus; Hock, Rainer; Wellmann, Peter (Materials Research Society Symposium - Proceedings, 2010)
CIS based chalcopyrite absorber materials are usually substituted in the cation and anion lattice to yield mixed pentanary crystals with the general composition Cu(In,Ga)(Se,S)(2) to achieve an optimised adaptation of the semiconductor bandgap to the terrestrial solar spectrum. Real-time investigations during the annealing of stacked elemental layers (SEL) of sputtered metals Cu and In and evaporated chalcogens S and Se with varying ratios were performed by angle-dispersive time-resolved XRD (X-ray diffraction) measurements. After qualitative phase analysis the measured powder diagrams were quantitatively analysed by the Rietveld method, the phases formed determined and their reaction kinetics obtained. Ternary indium and copper sulfoselenides form by the sulfoselenisation of the intermetallic alloy yielding different educts for the chalcopyrite formation with varying sulfur content. For S/(S+Se) >= 0.5 the formation of the chalcopyrite CuIn(S,Se)(2) is similar to the crystallisation path of CuInS(2). With increasing amount of selenium (S/(S+Se) = 0.25) different ternary sulfoselenides contribute to the semiconductor formation. For small amounts of sulfur, i.e. S/(S+Se) <= 0.1, the chalcopyrite crystallisation proceeds comparable to the one observed for sulfur-free Cu-In-Se precursors. The formation of CuIn(S,Se)(2) is accelerated and proceeds mainly after the peritectic decomposition of Cu(S,Se) to Cu(2)(S,Se). The sulfur content determines the crystallisation temperature of the semiconductor because Cu(S,Se) decomposes at higher temperatures with increasing sulfur. Upon heating S Se exchange reactions take place in the Cu-S-Se and Cu-In-S-Se syste
Keywords: phase transformation; mixture; thin film

Real-time measurement of the evolution of growth facets during SiC PVT bulk growth using 3-D X-ray computed tomography - Neubauer, Georg; Salamon, Michael; Uhlmann, Norman; Wellmann, Peter (Materials Science Forum, 2014)
In this paper, we present our technique for obtaining a real-time 3-D volume shape of the SiC crystal using X-ray computed tomography (CT). In particular, it is possible to determine in-situ the shape of the growth interface with high precision at growth temperatures above 2000 °C in a conventional 3 physical vapor transport (PVT) growth system. We show that the size and shape of a facet can be monitored at different stages during growth and furthermore the crystal's face boundary can be determined with high precision throughout the whole growth process. Real-time in our case means recording one image sequence within 2 to 15 minutes depending on the quantity and quality of the images. © (2014) Trans Tech Publications, Switzerlan
Keywords: 3-D X-Ray Computed Tomography; Bulk Crystal Growth; CT; Facet; In Situ Observation; PVT; Reconstructed 3-D Volume Data; Silicon Carbide (SiC); Sublimation Growth

Real-Time Study on Structure Formation and the Intercalation Process of Polymer: Fullerene Bulk Heterojunction Thin Films - Kassar, Thaer; Berlinghof, Marvin; Güldal, Nusret Sena; Schmutzler, Tilo; Zontone, Federico; Brandl, Marco; Metwalli, Ezzeldin; Will, Johannes; Li, Ning; Ameri, Tayebeh; Brabec, Christoph; Unruh, Tobias (Solar RRL, 2020)
Fullerene intercalation between the side chains of conjugated polymers has a detrimental impact on both charge separation and charge transport processes in bulk heterojunction (BHJ) organic photovoltaic cells (OPVs). In situ grazing incidence X-ray scattering experiments allow to characterize the structure formation, drying kinetics, and intercalation in blends of phenyl-c61-butyric acid methyl ester (PC60BM) and poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) named (pBTTT-C14) from their 1,2-orthodichlorobenzene (oDCB) solutions with different volume fractions of dodecanoic acid methyl ester (Me12) as a solvent additive. The structure formation process during evaporation of the solvent:additive mixture can be described by five periods, which are correlated to a multistep contraction of the lamellar stacking of the bimolecular crystals. The onset of crystallization is delayed by increasing the additive volume fraction in the coating solution leading to a promoted crystallinity. A conclusive picture of fullerene intercalation and additive-tuned structural evolution during the drying of thin films of the polymer:fullerene BHJ blends will be presented.
CRIS-Team WoS Importer:2020-02-04

Recent advances in semi-transparent polymer and perovskite solar cells for power generating window applications - Xue, Qifan; Xia, Ruoxi; Brabec, Christoph; Yip, Hin-Lap (Energy and Environmental Science, 2018)
Semi-transparent photovoltaic (ST-PV) technologies can be applied to replace facades and roofs in conventional buildings and coatings on vehicles to produce energy from sunlight. Current ST-PV technology is Si-based, but although Si achieves adequate efficiencies, it compromises on aesthetic appeal; its color is intrinsically difficult to tune. However, this presents an opportunity for semi-transparent polymer and perovskite-based PVs, the optical properties of which can be modulated easily by tuning their material compositions. In this review article, we summarize recent progress made in the material selection, optical engineering and device architecture design for high-performance, semi-transparent polymer and perovskite solar cells and discuss challenges for the commercialization of these semi-transparent solar cells for power-generating applications in windows.
Keywords: Cell engineering; Optical properties; Perovskite; Perovskite solar cells; Photovoltaic cells; Polymer solar cells; Polymers; Silicon solar cells

Recent Developments in Halide Perovskite Nanocrystals for Indirect X-ray Detection - Balitskii, Olexiy; Sytnyk, Mykhailo; Heiß, Wolfgang (Advanced Materials Technologies, 2024)
Metal halide perovskites are revolutionizing X-ray detection through a combination of low cost, solution processing, favorable optoelectronic properties, and high stopping power for high-energy ionizing radiation. While perovskite single crystals and polycrystalline wafers are considered direct X-ray converters, most medical X-ray applications are based on scintillators that shift high-energy radiation into the visible. Several materials are on the market, but demonstrations based on CsPbBr3 nanocrystals, possibly embedded in a matrix material or combined with organic molecules as luminescent species, highlight their competitiveness with established scintillators in terms of radioluminescence yield and transient behavior. Major hurdles that perovskite nanocrystal scintillators must overcome are environmental stability and toxicity. While there are still few examples of high-performance lead-free perovskite nanocrystal scintillators, microcrystalline perovskites are emerging with promising properties, reduced toxicity, and significant Stokes shifts to avoid reabsorption of emission in thick films. Thus, the near future of perovskite nanocrystal scintillator materials will primarily be the adoption of recipes for materials with proven properties in microcrystalline form. The nanocrystal colloidal solutions will facilitate the large-scale printing of homogeneous and scattering-free films to obtain high contrast and spatial resolution X-ray images by scintillation.
CRIS-Team Scopus Importer:2024-06-07
Keywords: halide perovskites; lead-free; nanocrystals; scintillators; Stokes shift

Recent Developments in Quantum Dots/CNT Co-Sensitized Organic Solar Cells - Soltani, Rezvan; Katbab, Ali Asghar; Ameri, Tayebeh (Journal of Material Sciences & Engineering, 2017)
Polymer solar cells (PSCs) are emerging alternative candidates to the standard silicone technology for green and renewable energy generation owing to their flexibility and solution processability. Bulk heterojunction (BHJ) organic solar cells (OSCs) based on conjugated semiconducting polymers as donor (D) and fullerene derivatives as acceptor (A) offer large D/A interfacial area, which overcomes the short exciton diffusion length. Although, recent advances in narrow band gap semiconducting polymers have led to the improvement in power conversion efficiency of organic photovoltaics (OPVs) beyond 10%, inefficient charge separation and low carrier mobility as well as negligible photon harvesting in near-infrared (NIR) and/or infrared (IR) region of the solar spectrum have still remained as bottle neck for ultimate performance of OPVs. Most PSCs only absorb the UV-visible part of the solar spectrum, leading to the low light harvesting efficiency. Hence, solution processed photoactive materials comprising nanostructured semiconducting inorganic quantum dots (QDs) as sensitizer have attracted great attention to improve energy conversion efficiency of the OPVs. This is attributed to the outstanding optoelectronic properties of QDs such as band gap tunability, potential NIR photons harvesting and multi exciton generation (MEG). However, the main shortcoming of inorganic QDs based OSCs is randomly hopping charge transport among discrete QD particles, which can be tackled through hybridization with one dimensional (1D) electrically conductive nanostructured materials such as carbon nanotube (CNT). By this way, CNT particles would behave as support for anchoring the light harvesting semiconductor QDs, leading to the enhancement of the exciton dissociation and charge transport towards the corresponding electrodes. Recently, manufacturing PSCs co-sensitized by QD loaded CNT has been shown as a promising direction to maximize performance of the device. This article reviews the recent developments in enhancement of OPVs‟ performance by utilization of high efficient light harvesting QDs and/or their hybrid with 1D CNT
Keywords: Organic solar cell; Quantum dot; Carbon nanotube; Ternary solar cell; Hybrid solar cell